Numbers of in vitro spontaneous IgG, IgM and IgA plaque-forming cells (PFC) as assessed by a modification of the protein A haemolytic plaque assay were determined in the blood and synovial fluid of patients with seropositive rheumatoid arthritis (RA) and compared with those of control groups. The total numbers of PFC were significantly higher in the peripheral blood of patients with active seropositive RA than in that of normal controls. In addition, most B lymphocytes in the synovial fluid of patients with active seropositive RA were active immunoglobulin (Ig) producers, whereas synovial fluid lymphocytes from patients with inactive seropositive RA and seronegative arthritis were not. In general, IgA PFC were relatively high in blood, whereas IgG PFC dominated in the synovial fluid. IgM PFC appear to be relatively low in blood and synovial fluid. However, a relative increase of IgG PFC was noted in the peripheral blood of patients with active RA. To test for polyclonality of the increased Ig synthesis, we tested the sera of patients and controls for the presence of polyclonal antibodies against sheep erythrocytes (SRBC) and SRBC modified by fluorescein isothiocyanate (FITC) and trinitrophenyl (TNP). No differences were observed with SRBC and TNP-SRBC agglutinin titres between patients and controls, but patients with RA had higher titres of FITC-SRBC agglutinins than normal sera. This finding supports the concept of a polyclonal nature of antibody production in RA patients.