Synovial fluid lymphocytes and paired peripheral blood lymphocytes from 28 patients with rheumatoid arthritis were analyzed by 2-dimensional flow cytometry. We found that the composition of the lymphocyte population is different in synovial fluid compared with that in peripheral blood, in that there were increased proportions of Ia+ Leu-2a+/Leu-15- (T cytotoxic) cells and Leu-3a+/Leu-8- (T helper) cells, together with marked decreases in Leu-2a+/Leu-15+ (T suppressor) cells and Leu-3a+/Leu-8+ (T inducer) cells. These findings suggest that the unique composition of synovial fluid lymphocytes might be the result of T cell activation by some mechanism that is not known.