He is a cheerful old farmer who jokes as he serves rice cakes made by his wife, and then he switches easily to explaining what it is like to cut open a 30-year-old man who is tied naked to a bed and dissect him alive, without anesthetic.

"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," recalled the 72-year-old farmer, then a medical assistant in a Japanese Army unit in China in World War II. "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."

Finally the old man, who insisted on anonymity, explained the reason for the vivisection. The Chinese prisoner had been deliberately infected with the plague as part of a research project -- the full horror of which is only now emerging -- to develop plague bombs for use in World War II. After infecting him, the researchers decided to cut him open to see what the disease does to a man's inside. No anesthetic was used, he said, out of concern that it might have an effect on the results.

That research program was one of the great secrets of Japan during and after World War II: a vast project to develop weapons of biological warfare, including plague, anthrax, cholera and a dozen other pathogens. Unit 731 of the Japanese Imperial Army conducted research by experimenting on humans and by "field testing" plague bombs by dropping them on Chinese cities to see whether they could start plague outbreaks. They could.

A trickle of information about the program has turned into a stream and now a torrent. Half a century after the end of the war, a rush of books, documentaries and exhibitions are unlocking the past and helping arouse interest in Japan in the atrocities committed by some of Japan's most distinguished doctors.