Interleukin 1 receptor antagonist (IL-1ra) has been found in glycosylated form in the urine of febrile patients or of children with rheumatoid arthritis, and in the supernatant of monocytes cultured in the presence of immune complexes. It blocks competitively the binding of IL-1 alpha and beta to their receptors. Produced amongst others by mononuclear cell lines and matured monocytes and alveolar macrophages, it prevents prostaglandin E2 and collagenase production by fibroblasts and synovial cells. In mice, IL-1ra improves survival after lethal endotoxemia. In this study, both natural and recombinant human IL-1ra (rhIL-1ra) were tested in an allogeneic T-cell reaction, and in mitogen- or antigen-induced lymphocyte proliferation. Neither the natural nor the rhIL-1ra blocked T-cell proliferation, but rhIL-1ra abolished the effect of exogenous IL-1 beta. This was not due to a loss of bioactivity of IL-1ra in culture, as the IL-1ra of the supernatant still completely inhibited 125I-IL-1 alpha binding to EL 4-6.1 cells and markedly reduced PGE2 production during antigen presentation. We conclude that IL-1ra alone, even at high concentrations, is not sufficient to block human T-cell proliferation in vitro.