Unit 731, Japan’s Horrific Human Experiments Program During World War II
Updated: May 21
Officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army, Unit 731 was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army during World War II. Located in the Pingfang district of Harbin, in the puppet state of Manchukuo (now Northeast China), the facility was the epicentre of some of the most grotesque and inhumane experiments ever conducted on human beings.
Unit 731 was established under the command of General Shiro Ishii, a microbiologist and a fervent advocate of biological warfare. Ishii persuaded the Japanese military of the strategic advantages of developing biological weapons, and thus, in 1936, Unit 731 was officially formed. The facility, which encompassed more than 150 buildings spread over six square kilometres, was shrouded in secrecy, its true purpose concealed under the guise of water purification and disease prevention research.
Human experiments involved intentionally infecting captives, especially Chinese prisoners of war and civilians, to disease-causing agents and exposing them to bombs designed to disperse infectious substances upon contact with the skin. There are no records indicating any survivors from these experiments; those who didn't die from infection were euthanized for autopsy analysis.
After human experimentations, researchers commonly used either potassium cyanide or chloroform to kill survivors.
According to American historian Sheldon H. Harris:
The Togo Unit employed gruesome tactics to secure specimens of select body organs. If Ishii or one of his co-workers wished to do research on the human brain, then they would order the guards to find them a useful sample. A prisoner would be taken from his cell. Guards would hold him while another guard would smash the victim's head open with an axe. His brain would be extracted off to the pathologist, and then to the crematorium for the usual disposal.
Yoshimura Hisato, a physiologist with Unit 731, had a particular fascination with hypothermia. In his investigations into limb injuries as part of the Maruta experiments, Hisato routinely immersed prisoners' limbs in ice-filled water until they were frozen solid, forming a layer of ice over the skin. An eyewitness recounted that when these frozen limbs were struck with a cane, they produced a sound akin to hitting a wooden plank.
Following the freezing process, Hisato experimented with various methods to rapidly rewarm the frozen limbs. Sometimes he poured hot water over them, at other times he held them near an open flame, and occasionally he left the subjects untreated overnight to observe how long it took for their blood to thaw the frozen limbs naturally.
Atrocities Committed
Nakagawa Yonezo, professor emeritus at Osaka University, studied at Kyoto University during the war. While he was there, he watched footage of human experiments and executions from Unit 731. He later testified about the playfulness of the experimenters:
Some of the experiments had nothing to do with advancing the capability of germ warfare, or of medicine. There is such a thing as professional curiosity: ‘What would happen if we did such and such?’ What medical purpose was served by performing and studying beheadings? None at all. That was just playing around. Professional people, too, like to play."
Prisoners were injected with diseases, disguised as vaccinations, to study their effects. To study the effects of untreated venereal diseases, male and female prisoners were deliberately infected with syphilis and gonorrhoea, then studied. Prisoners were also repeatedly subjected to rape by guards.
A special project, codenamed Maruta, used human beings for experiments. Test subjects were gathered from the surrounding population and sometimes euphemistically referred to as "logs" (丸太, maruta), used in such contexts as "How many logs fell?" This term originated as a joke on the part of the staff because the official cover story for the facility given to local authorities was that it was a lumber mill.
According to a junior uniformed civilian employee of the Imperial Japanese Army working in Unit 731, the project was internally called "Holzklotz", from the German word for log. In a further parallel, the corpses of "sacrificed" subjects were disposed of by incineration. Researchers in Unit 731 also published some of their results in peer-reviewed journals, writing as though the research had been conducted on nonhuman primates called "Manchurian monkeys" or "long-tailed monkeys."
The experiments conducted at Unit 731 were as diverse as they were depraved. Prisoners, euphemistically referred to as "logs" (maruta) by the personnel, were subjected to a litany of barbaric procedures:
Vivisection: Prisoners, often without anaesthesia, were dissected alive to study the effects of diseases on the human body. Limbs were amputated to observe blood loss, organs were removed to understand their functions, and some subjects had parts of their stomachs removed and the oesophagus reattached directly to the intestines.
The New York Times interviewed a former member of Unit 731. Insisting on anonymity, the former Japanese medical assistant recounted his first experience in vivisecting a live human being, who had been deliberately infected with the plague, for the purpose of developing "plague bombs" for war.
"The fellow knew that it was over for him, and so he didn't struggle when they led him into the room and tied him down, but when I picked up the scalpel, that's when he began screaming. I cut him open from the chest to the stomach, and he screamed terribly, and his face was all twisted in agony. He made this unimaginable sound, he was screaming so horribly. But then finally he stopped. This was all in a day's work for the surgeons, but it really left an impression on me because it was my first time."
Biological Warfare Testing: Victims were intentionally infected with deadly pathogens such as anthrax, plague, and cholera. The spread and effects of these diseases were meticulously documented. Some prisoners were exposed to bomb blasts containing biological agents to study infection patterns and lethality. Unit 731 were involved in research, development and experimental deployment of epidemic-creating biological weapons in assaults against the Chinese. Plague-infected fleas, bred in the laboratories of Unit 731 and Unit 1644, were spread by low-flying airplanes over Chinese cities. These operations killed tens of thousands with bubonic plague epidemics. An expedition to Nanjing involved spreading typhoid and paratyphoid germs into the wells, marshes, and houses of the city, as well as infusing them in snacks distributed to locals.
Frostbite Testing: In experiments designed to understand how to treat frostbite, prisoners were subjected to extreme cold. Their limbs were frozen and then thawed to study the progression and treatment of gangrene. Army Engineer Hisato Yoshimura was in charge of these experiments, Military personnel of the Unit referred to Yoshimura as a "scientific devil" and a "cold-blooded animal" due to his strictness and involvement in mass killings and inhumane scientific test, which included soaking the fingers of a three-day-old child in water containing ice and salt.
Naoji Uezono, a member of Unit 731, described in a 1980s interview a grisly scene where Yoshimura had "two naked men put in an area 40–50 degrees below zero and researchers filmed the whole process until [the subjects] died. [The subjects] suffered such agony they were digging their nails into each other's flesh." Yoshimura's lack of remorse was evident in an article he wrote for the Japanese Journal of Physiology in 1950 in which he admitted to using 20 children and a three-day-old infant in experiments which exposed them to zero-degree-Celsius ice and salt water.
Although this article drew criticism, Yoshimura denied any guilt when contacted by a reporter from the Mainichi Shimbun.
Syphilis and Rape: In an effort to understand the transmission of venereal diseases, prisoners were deliberately infected with syphilis through forced rape, often under the guise of examining wartime medical conditions. Female prisoners were forced to become pregnant for use in experiments. The hypothetical possibility of vertical transmission (from mother to child) of diseases, particularly syphilis, was the stated reason for the torture. Fetal survival and damage to mother's reproductive organs were objects of interest. Though "a large number of babies were born in captivity," there have been no accounts of any survivors of Unit 731, children included. It is suspected that the children of female prisoners were killed after birth or aborted.
While male prisoners were often used in single studies, so that the results of the experimentation on them would not be clouded by other variables, women were sometimes used in bacteriological or physiological experiments, sex experiments, and as the victims of sex crimes.
The testimony of a unit member that served as a guard graphically demonstrated this reality:
One of the former researchers I located told me that one day he had a human experiment scheduled, but there was still time to kill. So he and another unit member took the keys to the cells and opened one that housed a Chinese woman. One of the unit members raped her; the other member took the keys and opened another cell. There was a Chinese woman in there who had been used in a frostbite experiment. She had several fingers missing and her bones were black, with gangrene set in. He was about to rape her anyway, then he saw that her sex organ was festering, with pus oozing to the surface. He gave up the idea, left and locked the door, then later went on to his experimental work
Chemical Weapons Testing: Unit 731 also tested chemical weapons on living subjects. Prisoners were exposed to various gases, including mustard gas and cyanide, to evaluate their effects and develop countermeasures. Some of the agents tested were mustard gas, lewisite, cyanic acid gas, white phosphorus, adamsite, and phosgene gas. A former army major and technician gave the following testimony anonymously (at the time of the interview, this man was a professor emeritus at a national university):
In 1943, I attended a poison gas test held at the Unit 731 test facilities. A glass-walled chamber about three meters square [97 sq ft] and two meters [6.6 ft] high was used. Inside of it, a Chinese man was blindfolded, with his hands tied around a post behind him. The gas was adamsite (sneezing gas), and as the gas filled the chamber the man went into violent coughing convulsions and began to suffer excruciating pain. More than ten doctors and technicians were present. After I had watched for about ten minutes, I could not stand it any more, and left the area. I understand that other types of gasses were also tested there.
— Hal Gold, Japan's Infamous Unit 731, p. 349 (2019)
Unit 731 also tested chemical weapons on prisoners in field conditions. A report authored by unknown researcher in the Kamo Unit (Unit 731) describes a large human experiment of yperite gas (mustard gas) on 7–10 September 1940. Twenty subjects were divided into three groups and placed in combat emplacements, trenches, gazebos, and observatories. One group was clothed with Chinese underwear, no hat, and no mask and was subjected to as much as 1,800 field gun rounds of yperite gas over 25 minutes. Another group was clothed in summer military uniform and shoes; three had masks and another three had no mask. They also were exposed to as much as 1,800 rounds of yperite gas. A third group was clothed in summer military uniform, three with masks and two without masks, and were exposed to as much as 4,800 rounds. Then their general symptoms and damage to skin, eye, respiratory organs, and digestive organs were observed at 4 hours, 24 hours, and 2, 3, and 5 days after the shots. Injecting the blister fluid from one subject into another subject and analyses of blood and soil were also performed. Five subjects were forced to drink a solution of yperite and lewisite gas in water, with or without decontamination. The report describes conditions of every subject precisely without mentioning what happened to them in the long run. The following is an excerpt of one of these reports:
Number 376, dugout of the first area:
September 7, 1940, 6 pm: Tired and exhausted. Looks with hollow eyes. Weeping redness of the skin of the upper part of the body. Eyelids edematous, swollen. Epiphora. Hyperemic conjunctivae.
September 8, 6 am: Neck, breast, upper abdomen and scrotum weeping, reddened, swollen. Covered with millet-seed-size to bean-size blisters. Eyelids and conjunctivae hyperemic and edematous. Had difficulties opening the eyes.
September 8, 6 pm: Tired and exhausted. Feels sick. Body temperature 37 degrees Celsius. Mucous and bloody erosions across the shoulder girdle. Abundant mucous nose secretions. Abdominal pain. Mucous and bloody diarrhea. Proteinuria.
September 9, 7 am: Tired and exhausted. Weakness of all four extremities.
Low morale. Body temperature 37 degrees Celsius. Skin of the face still weeping.
— Man, Medicine, and the State: The Human Body as an Object of Government Sponsored Medical Research in the 20th Century (2006) p. 187
During the war, the activities of Unit 731 were a closely guarded secret. The Japanese government and military meticulously concealed the true nature of the unit's operations, and its existence was known only to a select few within the highest echelons of power. Even within the unit, personnel were often kept in the dark about the broader scope of their work, with duties compartmentalized to prevent leaks.
In the aftermath of Japan’s defeat in 1945, the horrors of Unit 731 began to surface. The Soviet Union conducted trials of some Japanese military personnel in Khabarovsk in 1949, revealing gruesome details of the experiments. However, the United States, seeking valuable research data, offered immunity to General Ishii and other key figures in exchange for their findings. Consequently, many perpetrators evaded justice, and the full extent of their crimes remained obscured for decades.
Japan’s acknowledgment of Unit 731 has been a contentious and gradual process. For many years, the Japanese government maintained a stance of denial or obfuscation regarding the unit’s activities. It was only in the 1980s and 1990s, under pressure from researchers, historians, and international bodies, that Japan began to confront this dark chapter of its past more openly.
In 1997, the Japanese court recognised the existence of Unit 731 and the suffering of its victims, although it stopped short of issuing a formal apology or providing compensation. Over the years, various documentaries, books, and academic studies have shed light on the atrocities committed, fostering a more nuanced understanding of the extent of Japan’s wartime conduct.
However, official apologies have remained insufficient for many survivors and their families. In 2002, a group of Chinese plaintiffs sued the Japanese government for compensation and an apology. The Tokyo District Court acknowledged the atrocities but ruled that individual victims had no right to seek compensation under international law.
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