Some results of long-term experiments performed in mice contaminated with 226Ra and 239Pu, especially the damage to the hemopoietic system and the incidence of osteosarcoma, are reported. Different degree of damage to the bone marrow hemopoiesis, depending on the arrangement of marrow cavities and the capability of stem cells to produce differentiated progeny are evaluated. This ability decreases with age and with duration of the contamination due to radionuclides. From the above aspects of radiation injury the ability of hemopoiesis to compensate the decreased production of differentiated peripheral cell forms is discussed. It is remarkable that myeloid leukemia occurred earlier and more frequently in 239Pu-contaminated animals than in controls. After the injection of plutonium-239, myeloid and lymphatic leukemias were diagnosed in most of the animals dying without osteosarcoma. These diseases represented a danger not lower than that of osteosarcoma which appeared in average later and the expected incidence of which was estimated to rise linearly with age. The findings are discussed from the aspect of their application in evaluating the risk resulting for man from the incorporated bone-seeking alpha emitters.