The effect of oral administration of coumarin on the induction of breast tumors brought about by DMBA has been studied in female Wistar rats. Coumarin given before DMBA caused a significant dose-related suppression of the incidence of adenocarcinomas, although no histologic difference was found in the tumor between untreated and coumarin-treated rats. The growth rate, size and multiplicity were also reduced and tumor occurrence was delayed. Parallel with the suppression, hepatic drug metabolizing activity was decreased and serum prolactin level increased. Coumarin given after DMBA elicited no effect on the carcinogenic potency of DMBA. In contrast to the action on mammary tumorigenesis, coumarin provided no protection against hemopoietic and adrenocortical necrosis brought about by DMBA.