Twelve naturally-occurring organosulfur compounds were investigated as inhibitors of cytochrome P450 1 (CYP4501)-mediated activation of benzo[ a] pyrene ( B[ a] P) in human hepatoma (HepG2) cells. Inhibition depended on the presence of a diallyl group and the number of S atoms. Diallyl trisulfide (DATS), with a diallyl group and three S atoms, had the highest activity with an IC50 of 0.4 mM, and 1.5-fold higher potency than diallyl disulfide ( DADS) containing a diallyl group and two S atoms. Organosulfur compounds containing an alkyl group were less effective, or even ineffective, inhibitors of both CYP450 1 and B[ a] P-induced cytotoxicity than DADS and DATS. Alliin and S-allyl cysteine containing the S-cysteinyl group had no inhibition.