The effect of various aflatoxin B1 concentrations on human peripheral blood lymphocyte blast-transformation reaction was studied. There was aflatoxin B1-induced dose-dependent inhibition of DNA synthesis in the lymphocytes stimulated by phytohemagglutinin. The aflatoxin B1 concentrations causing 50% decreases in DNA synthesis were defined by the regression method. (ID50). Judging from the ID50 values, the treatment of cultured lymphocytes with cytochrome P-448 inductor and lymphocytic exposure in the presence of toxin and hepatic microsomes of mice showed a 6-7-fold increase in the inhibitory action of aflatoxin B1. This indicates that the metabolism of aflatoxin B1 brought about by the cytochrome P-450-dependent monooxygenase system plays an important role in the action of aflatoxin B1 on human lymphocytes.