With the object to elucidate the role played by thymus and spleen in radiation leukemogenesis, effects of thymectomy and splenectomy on the induction of leukemia in RF mice by X-ray-irradiation were studied, dividing lymphocytic leukemia into thymic type and non-thymic type. As the result it has been clarified that thymectomy mainly inhibits the occurrence of the thymic type lymphocytic leukemia while splenectomy on the occurrence of the non-thymic type lymphocytic leukemia. As for the mechanism of this inhibition, a discussion was made on the significance of thymus and spleen as the organs that are constituted of the cells most susceptible to neoplastic transformation. After both thymectomy and splenectomy not a single mouse developed leukemia when exposed to X-ray-irradiation.