Clonal assays for erythroid progenitors (BFU-e and CFU-e) were used to study 20 patients with rheumatoid arthritis, 15 of whom had anaemia of chronic disease and five of whom were haematologically normal. The numbers of bone marrow BFU-e and CFU-e in the anaemic patients did not differ significantly from those in normal controls. Macrophages were removed from the bone marrow by a combination of adherence and buoyant density centrifugation over a sucrose gradient and the resulting fractions were cultured alone or together with autologous adherent cells in BFU-e assays. Co-culture with adherent cells significantly increased colony growth in both the controls and in seven of eight anaemic patients studied. Serum from 14 anaemic patients and from five non-anaemic patients was added to cultures of bone marrow or to control peripheral blood 'null' cells. Anaemic serum uniformly either inhibited or failed to stimulate BFU-e growth under these conditions. Serum from non-anaemic patients and from 10 healthy controls stimulated BFU-e growth from 'null' cells to an equal degree.