The effect of 800 roentgens whole-body X-radiation upon animals with a previously induced regenerative anemia was compared with the effect of the same dose on normal comparable animals. Detns. were made of the erythrocytes and reticulocytes/cu. mm. and Hb in g./100 ml. Normal irradiated animals developed an anemia that reached the maximum in 14 days. Animals with a regenerative anemia prior to irradiation developed no further anemia. The recovery time was similar for both groups. The mean reticulocyte value of a group of phenylhydrazine-injected, X-rayed animals was decreased below the normal control value on the 6th postirradiation day only. A phlebotomized group of animals responded to irradiation in a manner comparable to the phenylhydrazine-treated group. Histologic studies showed that erythropoietic tissue in bone marrow and spleen were wholly destroyed in 3 days in normal animals receiving 800 roentgens. A slow recovery began on the 7th to 9th day. The erythropoietic tissue was only partially destroyed in animals irradiated with 800 roentgens after treatment with phenylhydrazine or phlebotomy. The data indicate that erythroblast vulnerability to irradiation injury is not increased by increased mitotic activity and proliferation.