Erythropoietin is a glycoprotein hormone which has become accepted as the mediator of normal red cell production. We have developed a specific, highly sensitive and reliable radioimmunoassay for this hormone. This radioimmunoassay appears to meet all expectations in terms of responding appropriately to all physiological manipulations so far attempted. Physiological conditions which heretofore have been suspected of depressing erythropoietin levels, such as hypertransfusion, fasting hyperoxia and hypophysectomy have been observed to have decreased erythropoietin levels. Also appropriate increases in erythropoietin have been measured following hypoxia, bleeding or cobalt administration in a variety of laboratory animals. In addition to these animal studies, we have begun to use the erythropoietin radioimmunoassay on the serum of selected clinical states. Primary polycythemic patients have erythropoietin values in the normal range. However, patients with polycythemia of secondary origin have serum values well above the normal range. It is felt that the erythropoietin radioimmunoassay will play an important role, not only in studies concerned with the normal physiology of erythropoietin, but also will prove of diagnostic value in clinical hematology.