This study examined levels of tryptase, a specific mast cell product, in synovial fluid. Samples of synovial fluid from eight patients with rheumatoid arthritis and ten with other arthritides were measured in solid-phase immunoradiometric assays. Elevated concentrations of tryptase were present in samples from three patients with rheumatoid arthritis, one with psoriasis, and one with Reiter's syndrome. The data support the theory that mast cell activation is involved in the pathogenesis in some inflammatory joint diseases, but activation does not appear to be disease specific.