Objective. To delineate the phenotype and function of synovial T cells in rheumatoid arthritis (RA).Methods. T cells from normal subjects or from RA peripheral blood (PB), synovial fluid (SF), or synovial tissue (ST) were analyzed phenotypically and functionally.Results. RA SF and ST T cells were found to be markedly enriched in CD45RAdim, CD45RO+, CD45RB-mature memory cells, whereas in the PB, CD45RAbdght naive T cells were more frequent than CD45RO+ memory T cells, and only a minority were CD45RBm. SF and ST T cells proliferated less well and produced less interleukin-2 in response to mitogenic stimuli than did PB T cells. However, synovial T cells effectively promoted the production of Ig from normal B cells. Moreover, PB and synovial T cells differed in their capacity to down-regulate immunoglobulin production.Anti-CD3-stimulated PB T cells suppressed Ig production unless their proliferation was prevented with mitomycin C. In contrast, synovial T cells were potent helpers of B cell Ig production regardless of antecedent treatment with mitomycin C. To examine the relationship between the CD45RBdh phenotype and B cell help, CD45RBdi" T cells were sorted from PB. As opposed to