In order to clarify possible factors responsible for varying responses in uremic patients treated with recombinant human erythropoietin (rHuEPO), we determined the inhibitory effects of ten uremic sera on the erythroid progenitors (CFU-E) and erythroid bursts (BFU-E). We also measured plasma EPO titers, Fe, UIBC, ferritin, PTH-C, beta 2-microglobulin, and aluminum in all ten patients. The inhibitor of CFU-E but not BFU-E, was present in the serum of the single anemic patient whose recovery took longer after the administration of rHuEPO. He did not have such conditions as iron deficiency, excess of aluminum, or chronic inflammation. The remaining patients, who had no CFU-E or BFU-E inhibitors, were good responders to rHuEPO. In none of ten patients were there inhibitors of granulocyte-macrophage progenitors (CFU-GM) or any differences in the other parameters. Although the inhibitory factor of CFU-E can be overcome with a larger dose, its prior determination may be useful to set out minimal effective dose of EPO treatment.