Since interleukin-4 (IL-4) displays agonistic effects on both T and B cells, we studied whether this lymphokine is involved in rheumatoid synovitis, a disease characterized by intense T cell infiltration and B cell stimulation. Rheumatoid arthritis synovial fluids (RA SF) contained no (less than 15 pg/ml) or very low amounts (less than 25 pg/ml) of IL-4, as measured by a sensitive and specific enzyme-linked immunosorbent assay. No IL-4 was produced by unstimulated rheumatoid synovial membrane. RA SF were found to inhibit phorbol myristate acetate (PMA)-dependent proliferation of normal peripheral blood lymphocytes (PBL). An inhibitory fraction with an apparent molecular weight of 150 kd was isolated by gel filtration. The inhibitory fraction strongly blocked the proliferation of PBL induced by PMA, PMA + IL-2, or PMA + IL-4. However, this fraction was less effective in blocking the proliferation of PBL induced by PMA + IL-2 + IL-4. High levels of transforming growth factor beta (TGF beta) were found in these RA SF, and an anti-TGF beta antibody was able to partially reduce the inhibitory activity. RA SF were found to inhibit phytohemagglutinin-induced IL-4 production by PBL. These data indicate that IL-4, similar to other T cell lymphokines, cannot be detected in RA SF and that RA SF contains an inhibitory activity, related in part to TGF beta, which blocks mitogen-induced proliferation of PBL, at least in part through an inhibition of T cell-derived lymphokine release.