Aflatoxin B1 (AFB1) is frequently found in food and feedstuffs. It is a potent mutagenic and carcinogenic (e.g. hepatocellular carcinoma) mycotoxin produced by Aspergillus flavus, A. parasiticus and A. nomius. At the cellular level, after appropriate metabolism of AFB1 in cytochrome P450-dependent reactions, the resulting epoxide binds to DNA and induces point mutations. Using a Salmonella microsuspension assay we have examined: (1) mutagenic activity of AFB1 in the presence of the S9 fraction in TA98 and TA100 tester strains, (2) antimutagenic activity of ellagic acid (EA), rutin (RU) and psoralen (PS) against AFB1. The result of our investigations, was that the TA98 strain was more sensitive than the TA100 to mutagenic action of AFB1 and antimutagenic action of EA, RU and PS. Among the three compounds PS showed the highest anti-AFBl activity; 20 ng of PS inhibited the response to 10 ng of AFB1 by 90%, while the same amount of EA inhibited AFB1 by 82%, and RU was less potent still (only 60% inhibition).