Synovial fluid (SF) from rheumatoid arthritis (RA) patients were tested for their ability to inhibit the proliferative responses of normal peripheral blood mononuclear cells (PBM) to mitogens and interleukin-2 (IL-2). SF significantly inhibited the responses to concanavalin A (CON A) and phytohaemagglutinin (PHA), but significantly enhanced the responses to IL-2. Similarly, SF mononuclear cells (SFM) were hyporesponsive to CON A and PHA compared with autologous PBM, but hyper-responsive to IL-2. It is concluded that an IL-2 inhibitor in RA SF is unlikely to be the cause of SFM hyporesponsiveness to mitogens.