1. Castration of female mice between 15 and 22 days of age entirely prevented the occurrence of spontaneous adenocarcinoma of the breast, while the non-breeding control mice of the same strain showed a tumor incidence of 78.5 per cent. 3 of the spayed animals developed a spontaneous spindle cell sarcoma. This indicates that the influence of the ovary on the development of breast carcinomas is a specific one, since ovariectomy does not lead to a resistance against other types of spontaneous tumors. 2. Castration between 2 and 6 months of age led to a marked reduction of the tumor incidence, but did not entirely prevent the occurrence of mammary tumors. 3. It is concluded that the spontaneous mammary carcinoma of the mouse is due to a hereditary organ disposition, which remains latent in the absence of ovarian function, but which becomes manifest when a certain amount of ovarian hormone, corresponding to 5 to 30 estrus cycles, has acted on the breast tissue.