Unit 731, formally known as Manchu Detachment 731 and also referred to as the Kamo Detachment and the Ishii Unit, was a secret division of the Imperial Japanese Army. This unit specialized in biological and chemical warfare research, conducting deadly human experiments and producing biological weapons during the Second Sino-Japanese War (1937–1945) and World War II.
From 1936 to 1945, Unit 731 was responsible for the deaths of an estimated 14,000 victims. Additionally, its operations and related facilities caused infectious diseases that are believed to have resulted in over 300,000 fatalities.
The unit operated from its main base in Pingfang, a district in Harbin, which was the largest city in the Japanese-controlled puppet state of Manchukuo, now part of Northeast China. It also maintained branch offices across China and Southeast Asia.
Unit 731, established in 1936, carried out some of the most egregious war crimes committed by the Japanese military. The unit referred to its victims as “logs,” further dehumanizing them, and housed them in facilities called “log cabins.”
Experiments conducted included injecting diseases, inducing dehydration, testing biological weapons, using hypobaric chambers, performing vivisections, harvesting organs, amputations, and testing conventional weapons. Victims included kidnapped men, women (including pregnant women), children, and even babies born from the systemic sexual violence inflicted by staff members.
The majority of victims were Chinese, with a significant number being Russian, alongside others from different nationalities. Unit 731 also deployed biological weapons in unoccupied Chinese territories, contaminating cities, towns, water sources, and agricultural fields.
To eliminate evidence of their crimes, all prisoners in the compound were executed, leaving no survivors.
Unit 731 was initially established by the military police of the Empire of Japan but was later placed under the command of General Shirō Ishii, a combat medic officer, who led it until the war’s end. The facility, constructed in 1935, replaced the Zhongma Fortress, a prior prison and experimental camp, enabling Ishii and his team to expand their operations.
The program was heavily funded and supported by the Japanese government until its conclusion in 1945. Decades later, on 28 August 2002, the Tokyo District Court ruled that Japan had engaged in biological warfare in China, holding it accountable for the deaths of numerous residents.
After Japan’s surrender, both the Soviet Union and the United States collected data from Unit 731’s operations. Twelve researchers captured by Soviet forces were tried during the December 1949 Khabarovsk war crimes trials, receiving light sentences of two to 25 years in Siberian labor camps in exchange for the knowledge they possessed.
Meanwhile, researchers captured by the United States were granted secret immunity. The U.S. not only helped cover up the human experiments but also provided financial support to the perpetrators, utilizing their bioweapons research and experience for its own military programs, similar to Operation Paperclip. The Soviet Union also used documentation obtained from the Unit to develop a bioweapons facility in Sverdlovsk.
Formations
Japan began its biological weapons program in the 1930s, using the Geneva Protocol of 1925, which banned such weapons, as evidence of their potential effectiveness. Following its invasion and occupation of Manchuria in 1931, Japan chose the region as the base for Unit 731, leveraging the separation from their main islands and access to a large population of Chinese individuals for experimentation.
The Japanese military viewed the Chinese as expendable resources, believing this approach would provide an advantage in biological warfare. While most victims were Chinese, others came from various nationalities, and the facility itself operated as both a research center and a prison, with experimentation areas built around detention spaces for close monitoring.
Unit 731, founded in 1936, grew to include 3,000 staff members and 150 structures, with the capacity to hold 600 prisoners at a time for experiments. This infrastructure allowed researchers to perform a wide range of experiments while overseeing the conditions of their captive test subjects.
Unit 731 operated as a secret division of Japan’s Kwantung Army in Manchuria during World War II. Under the leadership of Lieutenant General Shirō Ishii, it was part of the broader Ishii Network, an initiative to develop biological weapons for the Imperial Army.
The Ishii Network was based at the Epidemic Prevention Research Laboratory, founded in 1932 at the Japanese Army Military Medical School in Tokyo. Unit 731, the first of several covert offshoots of this laboratory, served as both a field station and an experimental site to advance biological warfare techniques.
These experiments included the deployment of biological weapons on Chinese cities, directly violating the 1925 Geneva Protocol that prohibited the use of biological and chemical weapons. The deliberate secrecy surrounding Unit 731 and its associated units reflected the participants’ awareness of the inhumanity and illegality of their actions.
The Epidemic Prevention Research Laboratory was established under the leadership of Shirō Ishii after his two-year study of research institutions in the United States and Europe. With support from senior military officials, the lab was created to advance the development of biological weapons.
Ishii’s primary goal was to design biological weapons specifically targeting humans, leading to the formation of Unit 731 to focus on this objective. To further his work, Ishii also organized a covert research group called the “Tōgō Unit” for chemical and biological experimentation in Manchuria.
In 1936, Emperor Hirohito authorized the expansion of Unit 731 and its integration into the Kwantung Army as the Epidemic Prevention Department. At this stage, the organization was divided into two branches: the “Ishii Unit” and the “Wakamatsu Unit,” operating from a base in Xinjing.
From August 1940 onward, these branches were collectively renamed the “Epidemic Prevention and Water Purification Department of the Kwantung Army.” This name was often abbreviated to “Unit 731,” which became its widely recognized designation.
Colonel Chikahiko Koizumi, a key supporter of Shirō Ishii within the army, later served as Japan’s Health Minister from 1941 to 1945. Koizumi had been involved in poison gas research since 1915, joining a secret committee during World War I after witnessing Germany’s effective use of chlorine gas at the Second Battle of Ypres, which caused 6,000 deaths and 15,000 injuries among Allied forces.
Zhongma Fortress
Unit Tōgō initially operated out of the Zhongma Fortress, a prison and experimentation camp located in Beiyinhe, a village roughly 100 kilometers south of Harbin along the South Manchuria Railway. The camp’s prisoners included common criminals, captured bandits, anti-Japanese partisans, political prisoners, and individuals falsely accused by the Kempeitai.
Prisoners were provided with sufficient food, including rice or wheat, meat, fish, and occasional alcohol, to ensure they were in normal health at the start of experiments. Over time, they were subjected to blood loss, nutrient deprivation, and dehydration, with their declining health meticulously documented; some were vivisected or deliberately infected with plague bacteria and other pathogens.
In the autumn of 1934, a prison break exposed the facility’s existence, and a suspected sabotage-related explosion in 1935 further compromised its secrecy. As a result, Ishii shut down Zhongma Fortress and received approval to establish a larger facility in Pingfang, approximately 24 kilometers south of Harbin.
Other units
Alongside the establishment of Unit 731, a decree also led to the creation of other biological and chemical warfare units. These included the Kwantung Army Military Horse Epidemic Prevention Workshop, later known as Manchuria Unit 100, and the Kwantung Army Technical Testing Department, later referred to as Manchuria Unit 516.
Following Japan’s invasion of China in 1937, additional chemical and biological warfare units were set up in major Chinese cities under the title of Epidemic Prevention and Water Supply Units. These detachments included Unit 1855 in Beijing, Unit Ei 1644 in Nanjing, Unit 8604 in Guangzhou, and Unit 9420 in Singapore, all forming part of Ishii’s network, which employed over 10,000 personnel at its peak in 1939.
Japanese medical doctors and professors were drawn to join Unit 731 due to the opportunity to conduct human experimentation, which was rare, and the significant financial support provided by the Japanese Army. This combination of resources and freedom made the network a key driver of Japan’s biological and chemical warfare efforts.
Experiments
The military police and the Special Services Agency were tasked with sourcing victims to serve as test subjects for Unit 731. A team of physicians ensured that these individuals remained in good health, preparing them for experimentation.
Not all individuals sent to Unit 731 were subjected to experiments, as these tests were reserved exclusively for those deemed healthy. Once selected, maintaining the health of these individuals became a primary concern to ensure the experiments could proceed as planned.
Human experiments at Unit 731 involved deliberately infecting captives, primarily Chinese prisoners of war and civilians, with disease-causing agents. Victims were also exposed to bombs engineered to release infectious substances upon skin contact.
No evidence exists of survivors from these experiments, as those who did not succumb to infection were executed to allow for autopsy studies. Survivors of the experiments were typically killed using potassium cyanide or chloroform to facilitate further research.
American historian Sheldon H. Harris described the brutal methods used by the Tōgō Unit to obtain specific body organs for experimentation. If researchers, including Shirō Ishii, required a human brain for study, guards were instructed to secure a sample by removing a prisoner from their cell.
The victim would be restrained by guards as another guard shattered their skull with an axe. The extracted brain would then be sent to the pathologist for research, and the victim’s remains would be disposed of in the crematorium.
Nakagawa Yonezo, a professor emeritus at Osaka University, studied at Kyoto University during the war and witnessed footage of Unit 731’s human experiments and executions. He later testified about the seemingly frivolous motivations behind some of the experiments conducted by the researchers.
According to Nakagawa, many experiments had no connection to advancing germ warfare or medical knowledge and were driven by professional curiosity. He questioned the purpose of studying beheadings, describing such acts as mere “playing around,” noting that even professionals indulged in such senseless activities.
Prisoners at Unit 731 were injected with diseases under the guise of vaccinations to observe their effects. Male and female prisoners were also deliberately infected with syphilis and gonorrhea to study the progression of untreated venereal diseases.
A special project, codenamed “Maruta,” used humans as experimental subjects, often euphemistically referred to as “logs” in phrases like “How many logs fell?” The term “logs” originated from a staff joke, as the facility was officially described to local authorities as a lumber mill; internally, it was also referred to as “Holzklotz,” the German word for log.
The corpses of victims who were “sacrificed” during experiments were incinerated to conceal evidence. Some researchers published their findings in peer-reviewed journals, falsely claiming their work was conducted on “Manchurian monkeys” or “long-tailed monkeys” instead of human subjects.
At the age of 14, Hideo Shimizu joined Unit 731 as part of the fourth group of minors recruited, encouraged by a former school teacher. He recalled entering a specimen room filled with jars of various sizes, some as tall as an adult, containing preserved human body parts such as heads and hands.
Among the specimens was the body of a pregnant woman with her lower abdomen exposed, revealing a fetus with visible hair. Shimizu learned that prisoners were dehumanized by being referred to as “logs” and that their confinement facilities were called “log cabins.”
Vivisection
Thousands of men, women, children, and infants held in prisoner of war camps were subjected to vivisection, often conducted without anesthesia and resulting in death. Former Unit 731 member Okawa Fukumatsu admitted in an interview to vivisecting a pregnant woman.
Vivisections were carried out on prisoners after they had been deliberately infected with diseases. Researchers performed invasive surgeries to remove organs and study the impact of these diseases on the human body.
Prisoners at Unit 731 had limbs amputated to study the effects of blood loss, with some limbs reattached to the opposite side of their bodies. Others underwent surgeries where their stomachs were removed, and their esophagus was connected directly to the intestines, while organs like the brain, lungs, and liver were partially removed.
Imperial Japanese Army surgeon Ken Yuasa revealed that vivisection was a common practice among Japanese personnel even beyond Unit 731, with an estimated 1,000 individuals involved in such activities in mainland China. Yuasa admitted that the vivisections he performed were primarily for practice rather than research and described these acts as “routine” among Japanese doctors stationed in the region during the war.
The New York Times interviewed a former member of Unit 731, who shared their experience under the condition of anonymity. The individual, a former Japanese medical assistant, described their first vivisection on a live human, a prisoner intentionally infected with the plague as part of research to develop “plague bombs” for warfare.
Recalling the event, the assistant explained how the victim, understanding his fate, did not resist being led into the room or tied down. However, as the scalpel was picked up, the prisoner began screaming in agony as the assistant cut from the chest to the stomach, with the horrifying sounds leaving a lasting impression because it was their first vivisection.
Other accounts revealed that it was common practice in Unit 731 for surgeons to place a rag or medical gauze into the mouths of prisoners before performing vivisections. This was done to muffle their screams during the procedures.
Biological warfare
Unit 731, along with its affiliated units such as Unit 1644 and Unit 100, played a significant role in the research, development, and use of biological weapons. These units conducted experimental deployments of epidemic-causing agents in attacks against both Chinese civilians and military personnel during World War II.
By 1939, Shirō Ishii had identified six highly potent pathogens for biological warfare: anthrax, typhoid, paratyphoid, glanders, dysentery, and plague-infected human fleas. These pathogens were chosen for their ability to cause large-scale epidemics and their resilience to aerial dispersal.
Ishii then advanced to the next stage of his plan: field trials conducted through military operations targeting unsuspecting civilians. His methods included developing biodegradable bombs filled with live rats and fleas infected with diseases, designed to explode mid-air, allowing the infected creatures to safely reach the ground. Additionally, birds and bird feathers contaminated with anthrax were released from low-flying aircraft to spread the pathogens further.
Plague-infected fleas, bred in the laboratories of Unit 731 and Unit 1644, were dispersed over Chinese cities like Ningbo and Changde in 1940 and 1941 using low-flying airplanes. These operations triggered bubonic plague outbreaks, killing tens of thousands.
In Nanjing, typhoid and paratyphoid germs were introduced into wells, marshes, houses, and even snacks distributed to locals. Epidemics followed soon after, with researchers celebrating their findings and concluding that paratyphoid fever was the most effective pathogen for such operations.
The Library of Congress holds three declassified documents from Unit 731, each exceeding 100 pages, translated from Japanese to English. These records contain detailed clinical observations on the daily progression of various pathogens within the bodies of prisoners experimented on by Japanese doctors.
Testimonies from Japanese soldiers revealed that the program was capable of producing substantial quantities of biological agents each month. This included 300 kilograms of plague, 500–700 kilograms of anthrax, 800–900 kilograms of typhoid, and 1,000 kilograms of cholera, with even small amounts of these bacteria capable of causing devastating harm and fatalities.
Shirō Ishii concluded that fleas were highly effective carriers for transmitting plague, prompting Unit 731 to focus on mass-producing them. The unit operated approximately 4,500 flea incubators, each capable of producing at least 45 kilograms of fleas per cycle.
The large-scale production of plague bacteria and infected fleas, paired with the high mortality rates of plague infections, showcased the significant biological warfare capabilities of the Japanese military. Researchers at Unit 731 had the necessary resources to apply the scientific method in experiments involving inoculation and the development of airborne bacterial bombs.
Food items became a preferred method for delivering bacteria, with Unit 731 maintaining a supply of uncontaminated fruits for experiments. In one instance, typhoid bacteria were introduced into melons and cantaloupes, with bacterial density monitored until it reached a target level before the infected fruit was distributed to a small group of prisoners to spread the disease among the entire group.
Unit 731 also conducted field trials of biological warfare by targeting Chinese civilian populations. Between 1940 and 1943, researchers discovered that bacterial bombs were ineffective for transmission but achieved success using planes to spray microorganisms, a method later adopted by Unit 100 for similar operations.
Unit 731 conducted at least 12 large-scale biological weapon field trials, targeting a minimum of 11 Chinese cities with biological agents. One attack on Changde in 1941 resulted in around 10,000 biological casualties, including 1,700 deaths among Japanese troops, most of whom succumbed to cholera due to inadequate preparation.
Japanese researchers tested diseases such as bubonic plague, cholera, smallpox, botulism, and others on prisoners, advancing their understanding of these pathogens. This work led to the creation of weapons like the defoliation bacilli bomb and the flea bomb for spreading bubonic plague, with some bombs incorporating porcelain shells, an innovation proposed by Ishii in 1938.
These bombs allowed Japanese forces to conduct biological attacks, contaminating agriculture, reservoirs, wells, and other areas with fleas carrying anthrax or plague, as well as pathogens like typhoid and cholera. Researchers, dressed in protective suits, observed and documented the victims’ suffering and deaths during these experiments.
Infected food supplies and clothing were dropped by airplanes into Chinese regions not under Japanese control, while poisoned food and candy were distributed to unsuspecting individuals. Bombs containing plague fleas, infected clothing, and contaminated supplies were deployed on various targets, causing outbreaks of cholera, anthrax, and plague that are estimated to have killed at least 400,000 Chinese civilians. Tularemia was also tested on Chinese populations as part of these attacks.
In response to widespread accounts of Japanese biological warfare, Chiang Kai-shek dispatched a team of army and foreign medical personnel in November 1941 to gather evidence and treat victims. A report on the use of plague-infected fleas in Changde was released the following year, but it wasn’t until 1943 that Allied Powers addressed the attacks, with President Franklin D. Roosevelt publicly condemning them.
In December 1944, the Japanese Navy considered Operation PX, also called Operation Cherry Blossoms at Night, which aimed to launch biological attacks on California cities. Using Seiran aircraft deployed from Sentoku submarine carriers, the plan targeted San Diego, Los Angeles, and San Francisco with weaponized pathogens like bubonic plague, cholera, typhus, and dengue fever, while submarine crews would infect themselves and conduct suicide missions ashore.
Operation PX was finalized on March 26, 1945, but abandoned soon after due to objections from Chief of General Staff Yoshijirō Umezu. Explaining his decision, Umezu stated, “If bacteriological warfare is conducted, it will grow from the dimension of war between Japan and America to an endless battle of humanity against bacteria. Japan will earn the derision of the world.”
Weapons testing
Human subjects were used by Unit 731 to test the effectiveness of various weapons, including grenades placed at different distances and positions. Flamethrowers were also tested on live prisoners.
Victims were tied to stakes to serve as targets for pathogen-releasing bombs, chemical weapons, shrapnel bombs with varying fragment amounts, explosive devices, bayonets, and knives. To study treatments for shrapnel wounds suffered by Japanese soldiers, Chinese prisoners were strapped to wooden planks at varying distances from bomb blasts; survivors underwent surgery, while those who didn’t were subjected to autopsies.
Frostbite testing
Army Engineer Hisato Yoshimura conducted frostbite experiments by exposing captives to freezing conditions, often immersing their limbs in water of varying temperatures and allowing them to freeze. Once frozen, he struck the limbs with a stick, producing a sound “resembling a board being struck,” before chipping away the ice and subjecting the affected areas to different treatments.
Yoshimura was described by Unit personnel as a “scientific devil” and a “cold-blooded animal” for his ruthless methods, which included experiments on a three-day-old infant whose fingers were soaked in ice and salt water. Naoji Uezono, a Unit 731 member, later recounted a scene where Yoshimura forced two naked men into freezing conditions of -40 to -50 degrees Celsius, filming their agony until they died.
In a 1950 article for the Japanese Journal of Physiology, Yoshimura admitted using 20 children and an infant in experiments involving zero-degree-Celsius water and salt, but he denied guilt when confronted years later. He developed a “resistance index of frostbite” based on factors like the victim’s body temperature changes, environmental conditions, dietary intake, and physical activity prior to freezing, conducting detailed experiments to refine his findings.
Yoshimura’s studies involved varying experimental conditions, including the time of day, humidity, and pre-immersion treatments such as hunger, exercise, or different diets. These detailed parameters, meticulously recorded, contributed to his infamous legacy in frostbite research.
Syphilis
Unit 731 conducted syphilis experiments by forcing infected and non-infected prisoners into sexual acts to transmit the disease. According to testimony from a prison guard, researchers abandoned injecting the disease and instead brought infected males and females together in cells, threatening to shoot those who resisted.
After infection, victims were vivisected at various stages of the disease to observe the progression’s impact on internal and external organs. Female prisoners were blamed as hosts of the disease, with guards dehumanizing them by referring to their infected genitals as “jam-filled buns.”
Some children, born or raised within Unit 731, were also infected with syphilis and subjected to experiments. One Youth Corps member recalled seeing test subjects, including a Chinese woman holding an infant and two White Russian women with children, as researchers studied the effects of longer infection periods on treatment efficacy.
Rape and forced pregnancy
Female prisoners in Unit 731 were forcibly impregnated to study the possibility of vertical disease transmission, particularly of syphilis, from mother to child. Researchers also examined fetal survival rates and the damage caused to the mothers’ reproductive systems. Although many babies were born in captivity, there are no known accounts of any survivors, as it is suspected that children were either killed after birth or aborted.
While male prisoners were typically used for isolated studies to minimize experimental variables, female prisoners endured bacteriological and physiological experiments, sexual experimentation, and frequent sexual violence. A guard testified that some researchers used the time between experiments to assault female prisoners, describing one instance where a woman, already mutilated from frostbite experiments, was nearly raped but spared when her infection was discovered. This testimony underscores the pervasive cruelty and dehumanization inflicted on victims in Unit 731.
Other experiments
Prisoners at Unit 731 were subjected to horrific experiments, including deprivation of food and water to measure survival time and exposure to extreme low-pressure chambers that caused their eyes to protrude. They were also burned to study temperature effects, hung upside down until death, crushed under heavy objects, electrocuted, dehydrated with hot fans, or spun to death in centrifuges.
Other tests involved injecting prisoners with animal blood, exposing them to lethal X-rays, chemical weapons in gas chambers, or seawater injections, and even burning or burying them alive. Toxins such as tetrodotoxin, heroin, Korean bindweed, bactal, and ricin were tested on victims, while massive amounts of blood were drained to study blood loss effects, with at least half a liter removed every two to three days in some cases.
Dehydration experiments aimed to determine how long individuals could survive with minimal or no water intake, often after being starved beforehand. Victims’ physical deterioration was meticulously recorded, and in some cases, they were mummified alive under heat from dry fans, their corpses reduced to one-fifth of their normal body weight.
Unit 731 also conducted blood transfusion experiments using incompatible blood types. For instance, transfusions of A-type blood into O-type prisoners caused severe reactions such as increased pulse, fever, and malaise, while certain combinations caused no visible effects, as documented by Unit member Naeo Ikeda.
Unit 731 conducted extensive chemical testing on prisoners, including experiments with mustard gas, lewisite, cyanic acid gas, white phosphorus, adamsite, and phosgene gas. A building was specifically dedicated to gas experiments, where victims were subjected to lethal exposure to study the effects of these agents.
A former army major and technician recalled witnessing a gas test in 1943, where a blindfolded Chinese prisoner, tied to a post in a glass-walled chamber, was exposed to adamsite. As the gas filled the chamber, the man suffered violent coughing fits and severe pain, with over ten doctors and technicians observing the scene.
Takeo Wano, a former medical worker in Unit 731, reported seeing a Western man, believed to be Russian, preserved in formaldehyde after being vertically bisected. Unit 100 also experimented with toxic gas, using portable gas chambers resembling phone booths and testing the effects on prisoners wearing different levels of protection, from gas masks to no clothing at all.
Some experiments were described as excessively cruel and lacking any military justification, such as timing how long it took for three-day-old infants to freeze to death. These actions further exemplified the inhumane and sadistic nature of the research conducted within these units.
Unit 731 also conducted chemical weapons tests on prisoners under simulated field conditions. In one documented experiment conducted by the Kamo Unit from September 7–10, 1940, twenty subjects were divided into three groups and exposed to mustard gas (yperite) under various conditions.
One group, dressed in only Chinese underwear without hats or masks, was subjected to up to 1,800 rounds of mustard gas over 25 minutes. Another group, wearing summer military uniforms and shoes, with half given masks, faced the same exposure, while a third group, similarly dressed, endured up to 4,800 rounds of mustard gas.
Researchers observed the subjects at intervals of 4 hours, 24 hours, and 2, 3, and 5 days after exposure, documenting symptoms such as skin burns, eye irritation, respiratory distress, and gastrointestinal damage. Additional experiments included injecting blister fluid from one subject into another and forcing five prisoners to drink a solution of mustard gas and lewisite to study the effects on internal organs.
The detailed observations recorded symptoms like severe skin redness, swelling, blisters, and bloody diarrhea but omitted the long-term outcomes of the subjects. Reports included vivid descriptions, such as those of one prisoner experiencing severe edema, weeping skin lesions, abdominal pain, and profound exhaustion as their condition deteriorated.
Prisoners and victims
After Japan’s defeat in World War II, Unit 731 executed all prisoners to eliminate evidence of their atrocities. Approximately 300 prisoners were killed on August 11 and 12, 1945, with some coerced into suicide using ropes, while others were forced to ingest potassium cyanide or were killed via injections, likely of chloroform.
In 2002, a symposium in Changde, China, estimated that germ warfare and human experiments by the Japanese Imperial Army had caused around 580,000 deaths. Historian Sheldon H. Harris placed the death toll at over 200,000, with additional casualties caused by plague-infected animals released in the Harbin region after the war, resulting in outbreaks that killed at least 30,000 people between 1946 and 1948.
Victims of Unit 731 included political prisoners, captured bandits, anti-Japanese partisans, and impoverished civilians, as well as homeless and mentally disabled individuals, including infants, pregnant women, and the elderly. It is estimated that 3,000 men, women, and children were subjected to experiments in the Pingfang camp alone, though some accounts suggest this figure may have been as high as 10,000.
While the majority of victims were Chinese, others included Russians, Mongolians, Koreans, and a smaller number of Europeans, Americans, Indians, Australians, and New Zealanders. According to historian Sheldon H. Harris, most victims were political dissidents or those suspected of anti-Japanese activities, with Seiichi Morimura estimating that 70% were Chinese and nearly 30% were Russian.
Prison Facilities
Prisoners were transported in windowless, black vehicles with ventilation holes and were taken through secret tunnels to inner prison blocks. Men were held in Building 7, while women and children were kept in Building 8, with their health tested to ensure they were fit for experimentation.
Once deemed healthy, prisoners were stripped of their names and assigned numbers, which were recorded until their deaths. Cells had wooden floors, squat toilets, and small windows, and guards could patrol the outer walls. The inner-prison had strict security, requiring special permits for access and employing a “special team” that worked in full protective gear.
Escape Attempt
Despite the prison’s tight security, one failed escape attempt occurred. In one account, two Russian prisoners staged an ambush, overpowered a guard, and opened other cells, allowing several prisoners to roam the corridors.
Though some demanded to be shot rather than subjected to experiments, the escapees were ultimately killed. A witness admitted that staff members felt morally defeated by the bravery of the prisoners, acknowledging the injustice of their actions.
Experiments on Staff
Even Unit 731 staff members were occasionally subjected to experiments. Assistant Yoshio Tamura recalled that Yoshio Sudō, a staff member infected with plague during production, was vivisected by order of the Special Team.
Tamura described how Sudō, visibly deteriorated, was restrained and sanitized before being dissected alive, with his final word being “brute.” Youth Corps member Yoshio Shinozuka also testified that his friend Mitsuo Hirakawa was vivisected after contracting plague.
Known Unit Members
Prominent figures in Unit 731 included Shirō Ishii, the commander, and Ryōichi Naitō, who later founded Japan’s first commercial blood bank. Masaji Kitano, a major general, succeeded Ishii as commander in 1942.
In April 2018, the National Archives of Japan released a list of 3,607 members of Unit 731, further aiding research into its operations. Several members were tried and sentenced during the Khabarovsk War Crimes Trials, receiving prison terms ranging from two to 25 years.
Divisions and Facilities
Unit 731 consisted of eight divisions, each responsible for specific tasks, such as testing biological agents on prisoners, manufacturing biological weapons, and mass-producing bacteria. Its complex in Harbin spanned six square kilometers, housed over 150 buildings, and included facilities for raising fleas, producing chemicals, and storing biological agents.
Some satellite facilities remain in use as industrial sites, while others, like the main complex, have been preserved as museums. The unit also operated branches in Linkou, Mudanjiang, Hailin, Sunwu, and Hailar, and maintained a research facility in Tokyo, where bodies were buried after Japan’s surrender.
Surrender and Immunity
As World War II neared its conclusion, Unit 731 killed all remaining prisoners to conceal evidence of their activities, leaving no documented survivors. With the arrival of the Red Army in August 1945, orders were issued from Tokyo to destroy incriminating materials, including those at the Pingfang facility.
Roughly 300 prisoners were poisoned or gassed, while an additional 600 Chinese and Manchurian laborers were executed. To ensure secrecy, skeleton crews destroyed the compound, though some structures survived due to their sturdy construction.
Former member Hideo Shimizu recalled being instructed to burn victims’ remains in the courtyard and destroy leftover bones with explosives. Before departing, he was provided cyanide to use in the event of capture.
American Grant of Immunity
In September 1945, Lieutenant Colonel Murray Sanders arrived in Japan to investigate its biological warfare activities. Initially unaware of Unit 731, Sanders uncovered details after threatening to involve the Soviets, prompting Japanese officials to reveal a manuscript on biological warfare.
General Douglas MacArthur, Supreme Commander of the Allied Powers, negotiated a deal with Unit 731 members, granting them immunity from prosecution in exchange for exclusive access to their research. U.S. authorities censored communications and withheld evidence from the Tokyo War Crimes Tribunal, where the only mention of biological experiments was dismissed due to lack of corroboration.
Historians argue that the U.S. prioritized obtaining data over prosecuting war crimes, although the research was later deemed crude and ineffective by experts like Sheldon H. Harris and Murray Sanders. Unlike German physicians, who were prosecuted and publicized, Unit 731 perpetrators were protected, a disparity critics attribute to racial bias.
Separate Soviet Trials
While the Tokyo Tribunal ignored Unit 731’s crimes, the Soviet Union prosecuted 12 top leaders and scientists in the Khabarovsk War Crimes Trials in December 1949. Sentences ranged from 2 to 25 years in labor camps, though most defendants returned to Japan by the 1950s.
The U.S. dismissed the trials as propaganda, asserting they were a distraction from Soviet mistreatment of Japanese prisoners of war. However, evidence suggests Unit 731 members shared biological warfare data with the Soviets in exchange for leniency, enabling the construction of a biological weapons facility in Sverdlovsk.
Postwar Developments
During the U.S. occupation of Japan, Unit 731 members evaded prosecution, with MacArthur ensuring their research remained confidential. Former members, including Masami Kitaoka, continued experiments on Japanese prisoners and mental patients under the guise of vaccine development.
Chief Shiro Ishii, granted immunity in exchange for data, remained shielded, and much of Unit 731’s documentation was archived by U.S. authorities before being returned to Japan. This secrecy persisted, leaving many crimes unaddressed.
Media Coverage and Debate
Discussions of Unit 731’s activities began in Japan during the 1950s, with incidents like the death of an infant from E. coli infection linked to former members. Authors such as Shūsaku Endō and Seiichi Morimura brought attention to the atrocities, though some inaccuracies in their work drew criticism.
In the 1980s, direct testimony from former members, including Ken Yuasa, began surfacing, detailing vivisections and other experiments. Documentaries like Japanese Devils featured confessions from Unit 731 staff, further exposing the scale of the crimes.
Significance in Bio-Warfare Research
Unit 731’s research was the largest biological warfare program during World War II, surpassing efforts by other nations in scope and resources. However, historians criticized its lack of scientific rigor, with only a few experiments, such as those on mustard gas and frostbite, demonstrating reliable methodologies.
Despite the controversy, some researchers published findings on topics like epidemic hemorrhagic fever (EHF), openly acknowledging human experimentation. These reports highlighted the widespread acceptance of Unit 731’s activities within Japan’s medical community, even after the war.
Government Response
The Japanese government has been criticized for its reluctance to acknowledge Unit 731’s crimes. In 1983, historian Saburō Ienaga faced pressure to remove references to the unit from textbooks, though evidence later forced the Ministry of Education to back down.
While Unit 731 is mentioned in modern Japanese textbooks, its activities are rarely detailed, leaving much of the history unexplored. Efforts to uncover the truth continue, with researchers and activists advocating for greater transparency.
