Sequential changes in serum erythropoietin (sEPO) levels were measured by radioimmunoassay in six patients receiving autologous rescue (AR) and 11 patients receiving an allogeneic bone marrow transplant (BMT) for malignant disease. Longitudinal studies showed an inverse relationship between sEPO and haemoglobin levels in the autologous rescue and allogeneic transplant patients throughout the 130 d post-transplant study period. Early post-conditioning EPO responses were normal for the haemoglobin level in both groups, but after day 14 post-transplant, erythropoietin production in response to anaemia became impaired in one autologous rescue patient and eight of the 11 allogeneic transplant patients. There was no clear association between late impairment of sEPO production and conditioning therapy, infection, graft-versus-host disease, immunosuppressive therapy or serum creatinine. Blood transfusion requirements were similar for both groups in the first month after transplantation, but from days 31 to 90 post-transplant, BMT patients required an average of 5.5 units per patient compared with 1 unit per patient for the autologous group. Marrow transplant procedures do not affect early EPO responses but may diminish late responses. The potential value of exogenous rHuEPO in hastening engraftment and decreasing transfusion requirements, particularly for those patients who appear to have impaired EPO responses, remains to be shown by clinical trials.