Peripheral blood progenitor cell (PBPC) mobilising regimens that do not include cytotoxic drugs are needed if PBPC are to be used for allogeneic transplantation. We have studied the effects of erythropoietin on bone marrow and circulating haemopoietic progenitors. Eleven patients with untreated lymphoma received epoetin-alpha 300 or 450 iu/kg subcutaneously (sc) thrice weekly for 2 weeks. Their bone marrows and peripheral bloods were normal at entry. There were no differences between dose levels. Peripheral blood colony-forming cell (CFC) numbers increased fivefold over baseline (p = 0.003) including a sevenfold increase in GM-CFC (p = 0.003). CD34-positive cell numbers increased 4.6-fold (P < 0.01). Maximal CFC release was seen at days 5-8. No consistent change in megakaryocyte numbers was seen. The increase in numbers of cells capable of long-term haemopoiesis was not significant. The ratio of myeloid:erythroid cells in bone marrow reduced from 2.7 to 0.86 after erythropoietin treatment (p = 0.047). In bone marrow the only significant rise in CFCs was in erythroid progenitors. Erythropoietin alone resulted in a modest increase in PBPC that is unlikely to be useful for transplantation. The effects of erythropoietin in combination with other cytokines or with cytotoxic drugs remain to be explored.