Recombinant human erythropoietin (rhEpo), now available, has become increasingly more important for clinical use, e.g., in the treatment of anemia of chronic renal failure, and has been shown to reverse anemia in these patients. When patients with anemia of chronic renal failure were treated with rhEpo at dosages between 40 and 120 U/kg three times per week, the numbers of circulating erythroid burst-forming units (BFU-E) and granulocyte-erythrocyte-macrophage-megakaryocyte colony-forming units (CFU-GEMM) significantly increased during the first week of therapy. In contrast, the incidence of circulating granulocyte-monocyte CFU (CFU-GM) was not significantly altered.