In order to establish valid experimental systems for the study of breast cancer, an examination of the biochemical and morphologic characteristics of transplantable and carcinogen-induced mammary tumors in rats and human infiltrating ductal carcinomas of the breast was undertaken. A comparison of measurements of several selected enzymes, nucleic acids and lipids has demonstrated that the biochemical characteristics of the rodent carcinomas induced by a single dose of 7,12-dimethylbenz (a) anthracene are similar to those of human breast cancers studied. The importance of validating experimental models, in order to achieve a logical translation of information to the human disease, is discussed.