The appearance of mammary tumors in male mice following the injection of estrogenic hormones has been reported by a number of investigators. Comprehensive reviews of the literature have appeared in papers by Loeb et al. (1), Cook et al. (2), and Gardner (3). It is now well established that the incidence of these mammary adenocarcinomas varies with the strain of mice under investigation and with the type and amount of estrogen used. Thus, male mice from stocks in which there is a high rate of spontaneous mammary cancer in the females have a higher percentage of tumors following treatment than males from low-cancer strains. Treated females in some instances have not shown as high tumor incidence as treated males of the same strain.