Serum and synovial fluid (SF) from 68 patients with rheumatoid arthritis (RA) were studied for the presence of immune complexes (IC) and the results correlated with extraarticular features and/or disease activity. IC were measured by the 125I Clq binding assay (ClqBA) and with one detecting IgG, IgA, C3 or C4 in IC. Disease activity correlated significantly with IgG or IgA containing and Clq binding IC. The IgA containing IC were found only in 25% of the patients, including all but one case of rheumatoid vasculitis, but otherwise only in seropositive active RA. C3 and C4 IC did not correlated with disease activity, seropositivity or vasculitis. IC in serum did not correlate with SF levels, but C4 containing IC were more frequent in SF (60%) than in serum (30%). Thus serum IC did not reflect SF levels. Patients with vasculitis showed more IC in the sera than in SF.