Ninety one patients between the ages of 17 and 58 years undergoing histocompatible allogeneic transplants from sibling donors were entered into a double-blind randomised trial to evaluate the effect of human erythropoietin (rhu EPO) at a dose of 300 units per kg/day given thrice weekly by intravenous injection on erythropoiesis and on erythrocyte and platelet transfusion requirements. Dose was ceased when the haemoglobin exceeded 12g/dL and recommenced if the haemoglobin fell below 12 g/dL, at 150 units/kg/day. If the haemoglobin exceeded 12 g/dL on a further occasion, the dose of rhu EPO was not given. Patients received two units of erythrocytes when the haemoglobin dropped below 8.5g/dL and received platelet transfusions when the count dropped below 20 x 10(9)/L. Univariate analysis revealed a significantly higher reticulocyte count, haemoglobin concentration and bone marrow erythropoiesis after day 14 in the group receiving rhu EPO but this was not reflected in decreased erythrocyte transfusion (7 +/- 5 in controls versus 6 +/- 5 in rhu EPO group) or in platelet transfusions (11 +/- 7 in controls versus 11 +/- 9 in rhu EPO group). Hospitalisation in each group was the same (29 +/- 8 in the control group and 28 +/- 8 in the rhu EPO group). However, in the multivariate analysis, the administration of rhu EPO was associated with an 18% reduction in erythrocyte transfusion requirement when other variables were taken into account. No side-effects due to rhu EPO were detected in this study.(ABSTRACT TRUNCATED AT 250 WORDS)