The pathogenesis of the anaemia associated with rheumatoid disease is unclear. It has previously been shown that the degree of the anaemia correlates with the severity of the inflammatory disease and that serum from patients with arthritis inhibits erythropoiesis. This study was designed to examine whether interleukin 1 could be a mediator of the anaemia in rheumatoid arthritis. Radioimmunoassay of interleukin 1 beta in serum showed that patients with rheumatoid arthritis and associated anaemia had significantly higher interleukin 1 beta concentrations than patients with rheumatoid arthritis without anaemia. Pure recombinant human interleukin 1 alpha and interleukin 1 beta, in concentration ranges similar to those found in the arthritic patients, markedly suppressed the colony formation of the erythroid, but not the granulocyte-macrophage progenitor cells in cultures of normal bone marrow. Natural human interleukin 1 and recombinant interleukin 1 beta, but not interleukin 1 alpha, suppressed in a dose dependent manner the proliferation of the human erythroleukaemia cell line (HEL) in cultures, suggesting that the interleukin 1 effect is a direct one. The results show that interleukin 1 is a humoral inhibitor of erythropoiesis and suggests that interleukin 1 is involved in the development of anaemia in association with rheumatoid arthritis.