The authors investigated the influence exerted on some lymphocyte and granulocyte functions during treatment with human recombinant erythropoietin in five female patients with renal failure due to chronic interstitial nephritis, who were included in a regular dialyzation programme. The authors revealed a significant increase of the candidacidal capacity of phagocytizing cells of the peripheral blood stream and an increased capacity of blastic transformation of lymphocytes after stimulation with the mitogen phytohaemagglutinin. In these changes the improved oxygen metabolism may participate by a correction of anaemia, when this preparation is administered, but it cannot be ruled out that erythropoietin interferes by some hitherto unknown mechanism with immune processes.