Studies were made on the carcinogenic activity of butylated hydroxyanisole (BHA) in rats, mice, and hamsters and the effect of the antioxidants BHA, butylated hydroxytoluene (BHT), ethoxyquin (EQ), sodium L-ascorbate (SA), ascorbic acid (AA), sodium erythorbate (SE), propyl gallate (PG), and alpha-tocopherol, on two-stage chemical carcinogenesis in rats initiated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 1,2-dimethylhydrazine (DMH), diethylnitrosamine (DEN), 7,12-dimethylbenz(a)anthracene (DMBA), N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN), N-ethyl-N-hydroxyethylnitrosamine (EHEN), or N-methylnitrosourea (MNU). BHA clearly induced squamous cell carcinomas in both the rat and hamster forestomach. The tumorigenic action of crude BHA on the forestomach is largely due to 3-tert-BHA. In two-stage chemical carcinogenesis, BHA promoted MNNG or MNU-initiated forestomach and BBN- or MNU-initiated urinary bladder carcinogenesis and inhibited DEN- or EHEN-initiated liver and DMBA-initiated mammary carcinogenesis. BHT demonstrated promotion potential for urinary bladder and MNU-initiated thyroid carcinogenesis and inhibited DMBA-initiated ear duct carcinogenesis. EQ promoted EHEN-initiated kidney carcinogenesis and inhibited DMBA-initiated mammary and EHEN-initiated liver carcinogenesis. SA promoted forestomach and urinary bladder carcinogenesis and SE likewise enhanced urinary bladder carcinogenesis. alpha-Tocopherol inhibited ear duct carcinogenesis. No effects of any of the antioxidants on glandular stomach carcinogenesis were found. The results clearly demonstrated that antioxidants have different effects (promoting or inhibitory influences) depending on the organ studied and suggest the importance of a whole body approach to their investigation.