The relationship of serum immunoreactive erythropoietin to haemoglobin concentration was defined for 54 patients with rheumatoid arthritis (RA) and 41 patients with anaemia of varying aetiology (excluding pregnancy and renal insufficiency), not associated with RA. Significant inverse correlations between the logarithm of serum immunoreactive erythropoietin and the haemoglobin concentration were noted for the anaemic patients in both groups. The regression line for the RA patients had a similar slope, but a significantly lower y-intercept as compared to that for the non-RA patients. Erythropoietin levels were also significantly lower for the group of RA patients than for the group of non-RA patients when matched for comparable haemoglobin concentrations. These studies suggest that the erythropoietin response to anaemia in RA is intact but blunted relative to that for anaemia of other aetiologies. Lower levels of serum erythropoietin in anaemic RA patients may contribute to the pathogenesis of their anaemia.