[en] Leukemia, lymphoma, and mammary tumors induced in rodents by ionizing radiation appear, in many aspects, to be the counterparts to those in man as have been observed among the atomic-bomb (A-bomb) survivors. The emphasis of radiation-induced leukemogenesis was placed on the induction of lymphoid leukemia originating in the thymus (thymic lymphoma), which is most frequently encountered in irradiated mice. In addition to the well-defined factors governing the development and progression of the disease, recent developing interest on the etiological implication of retroviruses and cellular oncogenes is also discussed. Radiation-induced mammary carcinogenesis in rats is a suitable model for study of human breast cancer with regard to their hormone responsiveness. Our experimental observations using various radiations with different energies in combination with prolactin are described, emphasizing the key role of the promotion phase in carcinogenesis. The experimental system enabled us to demonstrate a high relative biological effectiveness (RBE) of fission spectrum neutrons, which may indicate strong contribution of fission neutrons in induction of malignant tumors among survivors