&NA;Animal bioassays for carcinogenicity are essential components of occupational health studies. Animal data that have been collected under controlled experimental conditions provide definitive information about the carcinogenic activities of individual substances or defined mixtures and their relative potencies in the test species. Such information serves as a frame of reference for clinical and epidemiologic studies, pointing to potential adverse health effects and to the types of substances that might produce them. This article alerts the occupational and environmental health communities to 20 substances that produced breast tumors, 13 substances that produced uterine tumors, and 8 substances that produced ovarian tumors in long‐term National Toxicology Program animal studies. Each of the substances also produced neoplasms at other body sites. Follow‐up studies of molecular measures of exposure and response in people and in animals will reduce the uncertainties of transspecies extrapolations.