We studied the expression of the Tac antigen, the transferrin receptor (Tfr-R), HLA class II antigens (DR, DQ, DP), CD30, and Act 1 on purified CD4+ and CD8+ cells isolated from synovial fluid (SF), synovial tissue (ST), and peripheral blood (PB) of patients with rheumatoid arthritis (RA) and with non-RA inflammatory arthritides (not ST). Subfractionated T cells of PB from healthy individuals served as controls. SF CD4+ cells from RA and non-RA arthritides expressed the Tac antigen much more frequently than corresponding CD8+ cells (54 and 58% versus 16 and 17%). In contrast, SF CD8+ cells of both patient groups expressed the HLA class II antigens rather more frequently than the corresponding CD4+ cells (88 and 68% versus 72 and 40%). Tfr-R expression was low on CD4+ and CD8+ SF T cells from both patient groups. SF T cells did not express CD30, and their expression of Act 1 did not differ from that of normal PB T cells. The RA ST findings were similar to those of RA SF. The overall expression of activation markers on PB T cells of patients was slightly higher than on those of normal controls, and the RA group was slightly higher than the non-RA group. The results show that intra-articular T cells in arthritis are activated and that CD4+ and CD8+ subsets differ in their expression of Tac antigen and HLA class II antigens. There were also similar patterns of activation markers on both CD4+ and CD8+ SF cells from RA and non-RA arthritis patients, suggesting that several types of arthritis display a similar immunopathogenesis in the joints.