The plasma erythropoietin (Epo) concentration was measured by radioimmunoassay in 75 patients with acquired aplastic anaemia. Overall there was an inverse relationship between the concentration of plasma Epo and the degree of anaemia. Plasma Epo concentrations were lower in patients who were sampled soon after diagnosis as opposed to those studied at later times. Although a decrease in the plasma Epo concentration was noted in all erythroid responders following immunosuppressive (IS) therapy or bone marrow transplantation (BMT), it was lower in patients undergoing BMT than in those receiving IS therapy for any given degree of anaemia.