''I was never told what they were for but it was quite obvious,'' he said. ''I was never under any illusion that it was for any purpose other than assassinating human beings.''

Earlier, Dr. Lourens had worked for a different front company that tried to make substances to reduce the fertility and virility of blacks. He said he had been told the drugs were for women who were fighting against the Angolan Government and were falling pregnant too often, a story even he found implausible.

Nevertheless, Dr. Lourens said, his laboratory took up on the project, experimenting on animals, particularly baboons. Among his jobs was to design a restraining chair for the baboons and a stimulator to collect semen. He said he also witnessed the testing of tear gas on caged baboons.

Another witness, Schalk van Rensburg, who worked at another of the front companies, Roodeplaat Research Laboratory, said it produced chocolates and cigarettes infected with anthrax, beer bottles containing botulism and sugar laced with salmonella. Included in a document titled ''List of Sales'' were 32 bottles of cholera culture.

The hearings have also explored what some commissioners have described as the ''underlying criminality'' of the projects, which apparently cost millions of dollars and ended up making millionaires of some of the people in charge.

One reluctant witness, Dr. Wynand Swanepoel, a dentist who said he was the managing director of the Roodeplaat lab but had no idea what the company was making, apparently invested about $10,000 in company stock and a few years later made almost $800,000 when the company was sold.

''You would just call that a good investment, I suppose?'' a commission officials asked Dr. Swanepoel today. ''Yes,'' he replied.