The detection of rheumatoid factors (RFs) in synovial membranes and fluids of patients with rheumatoid arthritis (RA) has suggested that local production of these antiimmunoglobulin autoantibodies may have a role in the pathogenesis of synovitis. To quantitate RF synthesis in the rheumatoid synovial membrane, 12 synovial specimens were obtained from patients with seropositive RA, 5 from patients with seronegative RA, and 6 from patients with other arthritides. Single cell suspensions were cultured, and supernatants were analyzed for IgG, IgM, IgG-RF, and IgM-RF by solid-phase radioimmunoassays. IgM-RF was detected in all of the 12 seropositive culture supernatants, and IgG-RF was detected in 8 of the 12. Addition of cycloheximide to the cultures resulted in a greater than or equal to 40% decreased in the amount of IgM-RF. A similar decrease in IgG-RF occurred in the 4 cultures in which the largest amounts of IgG-RF were detected. IgM-RF synthesis represented 7.3 +/- 0.7% (mean +/- SEM) of the total IgM produced, and IgG-RF represented 2.6 +/- 1.1% (mean +/- SEM) of the IgG synthesized in those cultures with detectable IgG-RF. Cultures of synovial membrane cells (SMC) from seronegative RA patients or patients with other arthritides did not contain detectable amounts of IgM-RF or IgG-RF. Selective synthesis of RF by seropositive synovium was suggested by the observation that the fractions of synthesized IgM with RF activity were greater in the SMC supernatants than in paired sera in all cases, and the fractions of IgG with RF activity were greater in the SMC supernatants of 3 of the 4 cases in which substantial amounts of IgG-RF were produced. Comparison of the percentages of newly synthesized IgM with RF activity in paired cultures of SMC and peripheral blood mononuclear cells similarly indicated selective synthesis of IgM-RF by the synovium. These results demonstrate active and selective synthesis of both IgG-RF and IgM-RF by seropositive SMC. However, RFs account for only a minor fraction of the total Ig produced.