ERYTHROPOIETINE is a protein, capable of stimulating erythrocyte production, which is found in the plasma of animals subjected to hypoxia or following the administration of cobalt. Sufficient data have been accumulated in the more common hematologic disorders to draw some tentative conclusions about its etiologic and therapeutic significance in clinical medicine. The highest concentrations of erythropoietine have been observed in diseases associated with erythroid aplasia. Substantial levels have been observed in congenital hypoplastic anemia, aplastic anemia, marrow aplasia due to chloramphenicol, and acute lymphocytic leukemia. In diseases associated with hemolysis and erythroid hyperplasia, such as Cooley's erythroblastic anemia and sickle cell disease, concentrations of erythropoietine in the plasma are not as high as in patients with similar hemoglobin levels due to aplasia.