[en] Mammary carcinogenesis in three different rat strains has been studied after single and fractionated irradiations with X-rays and monoenergetic 0.5, 4 or 15 MeV neutrons. The strains show considerable differences in susceptibility for the induction of cancer by radiation. In general linear dose-response curves have been observed for mammary tumourigenesis in the three rat strains for both X-rays and fast neutrons. Within the statistical precision of the data a dependence of the RBE on neutron dose cannot be recognised. The highest RBE values, varying between 7 and 15 for different types of tumours in the three rat strains have been observed for 0.5 MeV neutrons. These RBE values are lower in general than those observed by other groups for beams of comparable energy. (author)