Plutonium, radium, radiothorium, and mesothorium deposited internally have produced anemias in beagle dogs. This has been reported in semi-annual and annual progress reports of this laboratory since March, 1953. The pattern produced by plutonium and radium for the first year of exposure has been summarized (1). An associated hemolytic component has been observed in the anemia of chronic bone-marrow failure from other causes (2). The Crs5-tagged red-cell survival method was employed to evaluate a possible hemolytic factor in the anemia of chronic internal irradiation. The acute anemia from external X-ray is mostly the result of capillary leakage of red cells (3-5), but evidence of intravascular red-cell damage has also been demonstrated (6). In dogs studied here an anemia is produced by potent a-, f-, and y-emitters which deliver continuous radiation at close proximity to the bone marrow. The potent ionizing properties of a-irradiation delivered at a cellular level probably represent the chief mode of biologic damage of these isotopes. METHODS