The phenotypic characterization of enzymatically dissociated mononuclear cells in synovial membrane samples from multiple sites in two patients with rheumatoid arthritis (RA) were examined by fluorescence activated flow cytometry. In synovial membrane samples from each patient there was a consistent increase in the proportion of CD8+ cells (suppressor/cytotoxic), CD14+ cells (monocytes/macrophages), and HLA-DR+ cells compared with paired peripheral blood mononuclear cells. The proportion of CD4+ cells (helper/inducer) in synovial membrane was variable. Studies of in vitro production of IgM and IgM rheumatoid factor in one patient showed strikingly similar values for synovial membrane rheumatoid factor production at the two sites, which was enhanced compared with production in peripheral blood. These results suggest that in individual patients with RA the intra-articular immune response is comparable at multiple anatomical sites and that it is distinct from that in peripheral blood.