The phenotypic characteristics of enzymatically dissociated synovial membrane mononuclear cells from 8 patients (14 samples) with rheumatoid arthritis (RA) were assessed by fluorescence activated flow cytometry and compared to peripheral blood (PB) mononuclear cells from 18 patients with RA and 14 normal controls. There was no significant difference between the percentage of CD4+ and CD8+ lymphocytes in synovial membrane compared to RA and normal PB. Double labelling experiments revealed similar percentages of CD4+ CDw29+ (helper-inducer) and CD4+ CD45R+ (suppressor-inducer) cells in RA and normal PB. In contrast, SM CD4+ CDw29+ cells were present in significantly higher proportions than in RA and normal PB (p less than 0.001). Conversely, synovial membrane CD4+ CD45R+ cells were present in significantly lower proportions than in RA and normal PB (p less than 0.001). A similar pattern of CDw29 and CD45R antigen expression was noted on CD8+ lymphocytes reflecting increased killer-effector (p less than 0.001) and decreased suppressor-effector (p less than 0.001) cells, respectively. Other experiments revealed a significant increase in the percentage of synovial membrane CD20+ cells (B lymphocytes) and HLA-DR+ cells compared to RA PB (p less than 0.02 and p less than 0.001) and normal PB (p less than 0.01 and p less than 0.005), and similar proportions of CD14+ cells (monocytes/macrophages). Our results suggest that RA synovial membrane contains populations of T and B lymphocytes that differ quantitatively and qualitatively from those in PB. These may account for some of the abnormalities in intraarticular humoral and cellular immune responses in patients with RA.