If i remember correctly they were also experimenting with bubonic plague as a biological weapon and nearly made an attack on san fransisco with it but disagreement from the navy prevented it.
They did some experimenting alright. The sick pecker heads had a facility in Manchuria where they infected people then cut them open to see how the illness effected living tissue. No anesthesia just your numbers up and you get filleted alive. The victims were refered to as "logs" because their dead bodies would be stacked like firewood before incineration. Too bad the political climate after the war kept this kind of information out of the limelight.
Unit 731 is the most notorious. Why we held war crime trials for the Nazis but granted immunity
to the Unit 731 crew for their research, I'll never know...
First, we held extensive war crimes trials in Tokyo following the war. If you have a sufficiently strong stomach, I would recommend reading Knights of Bushido, which is a summary of the trials.
With respect to Unit 731, the head was given a pass by the US because the Russians captured the facility in Manchuria where the unit was based. We needed to know what the Russians had captured. There is an extremely interesting document in the US National Archives that is an intelligence assessment submitted by the OSS in July of 1945, detailing the Japanese biological warfare program. Much of the information came from the interrogation of an individual who had a degree in bacterioiogy and was assigned to the unit, but was drafted by the Japanese Army and sent as a superior private in a combat unit to Peleliu. He was more than happy to surrender to the Marines, and sang like a little bird to his interrogators. His information was quite accurate. The report also includes an interesting report on German-Japanese information exchange on biological weapons, and on Japanese testing of various biological agents in China on the Chinese. Between the contents of the assessment, and the capture by Russia of the facility in Manchuria, I with extreme reluctance agree to giving the head of the program a pass. I really would not have wanted to be the person who had to make that decision.