HLA-DR expression was found on 6.7 +/- 0.7% of blood T lymphocytes from 34 patients with rheumatoid arthritis (RA) and 2.6 +/- 0.3% of T cells from normals. In synovial fluid (SF) of RA patients high percentages were found (56 +/- 3%, n = 18); the SF T lymphocytes showed an acid alpha-naphthyl acetate esterase pattern and cell morphology compatible with activated T lymphocytes. The helper function for pokeweed mitogen induced B cell differentiation was higher in blood T lymphocytes from RA patients than from normal controls; in SF T lymphocytes helper function could not be established. These differences between blood and SF T lymphocytes may represent differences in recirculating and homing properties of T cells in these compartments.