To evaluate the level of lymphocyte activation in reactive and rheumatoid arthritis, density gradient-isolated, synovial fluid mononuclear cells were stained with a panel of antisera directed at lymphocyte activation markers using an avidin-biotin-peroxidase complex (ABC) method. More specifically, we studied the expression of immune response-associated class II HLA antigen (Ia), of receptors for interleukin 2 (Tac) and transferrin (T9), as well as of gp 40/80 glycoprotein (4F2). Although Ia+ cells formed about 60% of all the synovial fluid mononuclear cells in both disease conditions, the proportion of Tac+ (33 +/- 4% vs 3 +/- 1%, P less than 0.001), T9+ (34 +/- 4% vs 5 +/- 2%, P less than 0.001), and 4F2+ (48 +/- 6% vs 3 +/- 2%, P less than 0.001) cells was high only in reactive arthritis. All the patients who had reactive arthritis followed a favourable clinical course during the 4-month-long prospective follow-up, whereas disease activity was stable in patients with rheumatoid arthritis. These findings suggest that the diseased joints in reactive arthritis are a site for an active, but normally down-regulated, cell-mediated immune response.