Unit 731 was an Imperial Japanese Army research and development facility for biological weapons, headquartered in Pingfan, Manchuria. It was headed by Gen. Shiro Ishii, who was given immunity from war crimes prosecution in exchange for his experimental data.
The program was always under the cover identity of a water purification laboratory, where Ishii had indeed done significant development. Ishii's first facility, built in 1932, was the Zhoghma Fortress, a prison on the outskirts of Harbin. After an escape in 1935, they moved closer to Harbin at Pingfang. Unit 731 also had a number of field subunits and specialized laboratories at other locations.
While there is no hard documentary evidence that Emperor Hirohito knew of the specific programs, he approved the general formation of the units. Bix suspects that he did, as he was in the habit of asking sharp questions about documents he was to sign, and also was a trained biologist who would have particular interest in scientific programs. He was routinely shown Kwangtung Army directives about operations, and that organization was responsible for biological warfare operations between 1940 and 1942.  There are conflicting reports if it was visited by Prince Chichibu.
Imperial General Headquarters denied knowledge of it, but there is evidence that the generals in charge of resource allocation knew of it. Unquestionably, senior officers of the semi-autonomous Kwangtung Army, with whom it conducted combat operations, knew of it. 
It engaged in human experimentation and field use of weapons, including attacks on 11 Chinese cities. 
Human experimentation, completely involuntary and in violation of the principles of the Nuremberg Code and the Declaration of Helsinki, was not limited to biological weapons. It also included experiments in conditions of freezing, extreme heat, and exhaustion; surgical procedures on living subjects without anesthesia; poisoning; and bullet and grenade tests. Some very similar experiments resulted in death sentences for Nazi experimenters at the Medical Case at the Nuremberg Military Tribunals
Most of the victims were Chinese, dehumanized as maruta or "logs" by the staff. The estimates of victims at Pingfan proper start at 1,000 and go upward. Slightly less than 1,000 human autopsies apparently were carried out at Unit 731, most on victims exposed to aerosolized anthrax. Many more prisoners and Chinese nationals may have died in this facility - some have estimated up to 3,000 human deaths.
In comparison with the contemporaneous U.S. and British programs, the Japanese methods for distributing infectious agents was relatively primitive. Interestingly, however, some of the devices alleged to be U.S. biological munitions, in the Korean War, had a significant resemblance to Unit 731 devices. The Soviets did capture some Unit 731 personnel, and that information may have made its way to North Korea.
When a disease was vector-borne, such as plague carried by fleas, the initial Japanese approach was to distribute infected arthropods, rather than the more difficult process of forming an aerosol spray. Anthrax was first delivered on bullets or bomb fragments. Plague was also delivered by rats in porcelain containers dropped by parachute.
Diseases being studied included:
In 1940, a plague epidemic in China and Manchuria followed reported overflights by Japanese planes dropping plague-infected fleas. Other flea attacks were reported over Yunnan Province bordering Burma, over Ningbo in eastern China and over Changde in north-central China. "In all, tens of thousands, and perhaps as many 200,000, Chinese died of bubonic plague, cholera, anthrax and other diseases." Later, aerosol attacks were used, which also caused Japanese casualties.
U.S. intelligence first obtained fragmentary knowledge of Unit 731 in December 1944, with more details in April 1945. After the surrender of Japan, a U.S. Army microbiologist, was sent to interrogate persons in the program and evaluate documents.
Two separate organizations were involved in the intelligence interrogations and document examinations: the Chemical Warfare Service, which provided the technical expertise, and the G-2 Division of Douglas MacArthur's Supreme Headquarters Allied Powers (SCAP), headed by Major General Charles Willoughby. The initial technical experts were Lieutenant Colonel Arvo Thompson and Dr. Norbert Fell.
Tsuneini Keiichi, a professor at Kanagawa University, found two significant documents from MacArthur's intelligence chief, Major General Charles Willoughby, dealing with the Japanese biological warfare research and operations by Unit 731. One, titled "report on bacteriological warfare" for the chief of staff of the Far Eastern Commission, dated July 17, 1947,  The other, dated July 22,went to Major General SJ Chamberlin, director of intelligence of the US War Department General Staff, to illustrate the need for continued use of confidential funds without restrictions to obtain such intelligence.
Willoughby described the achievements of his unit's investigations, saying the "information procured will have the greatest value in future development of the US BW (bacteriological warfare) program". Citing a US War Department specialist in charge of the investigation, Willoughby wrote in the report that "data on human experiments may prove invaluable" and said the information was "only obtainable through the skilful, psychological approach to top-flight pathologists" involved in Unit 731 experiments. It has also been suggested that G-2 may have exaggerated the unquestionable atrocities, possibly to obtain more funding. 
Tsuneishi said it had been thought that the US had gathered the information by making unit members choose between cooperating or facing war crime charges, "but it has become clear that this was done by winning (unit members') hearts with money and rewards". In 1947, SCAP G-2 tried to obtain the experimental data but "because war crime charges against the Unit 731 officers had been waived by then, the GHQ was apparently forced to offer monetary rewards to access the information." 
- Herbert P. Bix (2001), Hirohito and the making of modern Japan, Harper Perennial, ISBN 978-0060931308, pp. 362-364
- Sheldon H. Harris, Chapter 16: Japanese Biomedical Experimentation during the World War II Era, in Dave E. Lounsbury, Ronald F. Bellamy, Military Ethics, Volume 2, Borden Institute, U.S. Army Medical Department, pp. 467-469
- Biological Weapons Program, Federation of American Scientists
- Ed Regis (1999), The Biology of Doom: the History of America's Secret Germ Warfare Project, Henry Holt, ISBN 0805057641, pp. 85-86
- "US paid for Japanese human germ warfare data", ABC News Online, 15 August 2005
- Tsuneishi Keiichi (18 August 2005), New Facts about US Payoff to Japan’s Biological Warfare Unit 731, Kyodo News Agency; first appeared in Kanagawa Shimbun