Interleukin-1 (IL-1) is a macrophage derived mediator whose properties suggest that it could play a role in the pathology of arthritis. To test this hypothesis, joint fluids from patients with several different arthritides were tested. Small amounts of IL-1-like activity were recovered from many of these joint fluids after affinity chromatography over a column of rabbit anti-human IL-1. Positive fluids were obtained from patients with rheumatoid arthritis, psoriatic arthritis, Reiter's syndrome, osteoarthritis, gout, and traumatic arthritis. Upon gel filtration, the joint derived factor displayed a molecular weight distribution similar to that of IL-1 derived from human monocytes stimulated in vitro. These results suggest that IL-1 is present in joint effusions and, therefore, might contribute to joint destruction.