We examined the levels of TcR delta 1+ T cells (total gamma delta T cell) and delta TCS1+ (gamma delta T cell subset) T cells in the peripheral blood (PB) and synovial fluid (SF) of 16 patients with rheumatoid arthritis (RA) and compared them to the levels in PB of patients with Felty's syndrome (FS) and 21 healthy control subjects (NML). Synovial fluid from eight patients with seronegative spondyloarthropathies (SSA) was also examined. The results demonstrated elevated levels of the delta TCS1+ subset in the PB of RA and FS patients relative to NML (P less than 0.05). No such differences were observed in the levels of PB TcR delta 1+ T cells. The results did not appear to reflect a non-specific inflammatory response since delta TCS1 T cells were elevated in the SF of RA patients relative to SSA SF and NML PB. delta TCS1 T cells in SSA PB and SSA SF were comparable to NML PB. TcR delta 1+ T cells levels in RA SF were higher than SSA SF levels but were comparable to those of NML PB. Taken together, the results support a pathogenic role for delta TCS1+ T cells in RA.