Sera and synovial fluids from 88 patients with rheumatoid arthritis were examined for circulating immune complexes by three assays: monoclonal rheumatoid factor radioimmunoassay, C1q binding assay, and Raji cell radioassay. Paired samples were available for 82 patients. Immune complexes were detected with high frequency in the synovial fluid by each assay (75% by the monoclonal rheumatoid factor radioimmunoassay, 95% by the C1q binding assay, and 61% by the Raji cell readioassay). In rheumatoid arthritis sera, immune complexes were detected with high frequency by the C1q binding assay (85%) and the monoclonal rheumatoid factor radioimmunoassay (70%) but infrequently by the Raji cell radioassay (26%). The presence of immune complexes in serum was most frequently accompanied by the presence of complexes in fluid, regardless of the method of detection; moreover, the levels of immune complexes in synovial fluid were generally higher than in paired serum. Further, the levels of immune complexes as measured by the C1q binding assay correlated with certain parameters of clinical activity, while the monoclonal rheumatoid factor radioimmunoassay and Raji cell radioassay correlated with extraarticular features (excluding nodules) of rheumatoid arthritis.