Solid phase radioimmunometric methods have been used to assay immune complexes and IgG and IgM antiglobulins in paired samples of synovial fluid and serum from patients with rheumatoid arthritis (RA) or osteoarthrosis. Over 60% of RA patients had some increase in complexes in their sera, while nearly 90% had synovial fluid complexes. Moreover, the levels of complexes within the joint were much higher than in the serum. Both IgG and IgM antiglobulins were raised in most RA patients.The levels of IgG antiglobulins--and to a less extent IgM antiglobulins--were nearly always higher in synovial fluid than in the corresponding serum sample.A strong correlation was found between the levels of immune complex and IgG antiglobulin. A marked association was seen between the presence of subcutaneous nodules and increased IgG antiglobulins.