The effects of ethinyl estradiol (EE) on 3,2'-dimethyl-4-aminobiphenyl (DMAB)-induced carcinogenesis were examined in Syrian golden hamsters. DMAB was subcutaneously injected in corn oil at a concentration of 100 mg/kg once a week for 20 weeks and EE was administered in the diet at a dose of 0.75 ppm throughout the experiment. Some animals were killed at week 20 and all surviving ones were killed at week 50. Gallbladder tumors (adenomas and carcinomas) were induced in 6 of 15 hamsters (40%) in the DMAB + EE group and 5 of 14 (36%) in the DMAB alone group in males, and in 6 of 13 (46%) in the DMAB + EE group and 1 of 8 (13%) in the DMAB alone group in females at week 50. A clearer enhancing effect of EE on DMAB gallbladder carcinogenesis was observed for tumor multiplicity (No./animal) for both sexes; from 0.36 to 0.67 in males and from 0.14 to 0.62 in females. Thus, DMAB was demonstrated to be carcinogenic in the gallbladder of hamsters and EE enhanced this DMAB-induced gallbladder tumorigenesis.