The mutagenicity of benzo(a)pyrene (BP) on Chinese hamster V-79 cells cocultivated with X-irradiated hamster embryo cells was inhibited by phenolic antioxidants, such as tert-butyl-4-hydroxyanisole (BHA) and butyl-hydroxytoluene (BHT), but not by disulfiram and its related compounds such as tetraethylthiuram disulfide, diethyldithiocarbamic acid and dimethyldithiocarbamic acid. The mutagenicity of BP on V-79 cells was reduced 44% by BHA and 25% by BHT. BHA inhibited BP-induced mutagenesis, but not N-acetoxy-2-acetylaminofluorene (N-acetoxy-AAF)-induced mutagenesis. BHA is suggested to inhibit the mutagenic action of BP by altering its metabolism.