We reported before that monosodium urate (MSU) crystals were potent stimulators of endogenous pyrogen (EP) production from human and rabbit mononuclear phagocytes, and proposed that this property of MSU crystals may be important in the pathogenesis of gout. EP activity is now attributed to interleukin 1 (IL 1) peptides but IL 1 is not the only pyrogenic monocyte-derived cytokine, since both interferon-alpha (alpha-IFN) and tumor necrosis factor (TNF) are also pyrogenic in rabbits. Using a T cell comitogenic assay based on a murine helper T cell clone that does not respond to IFN or TNF, we now report the release of IL 1 activity from human blood monocytes and synovial fluid mononuclear cells (MNC), following stimulation with MSU crystals. MSU-induced supernatants with IL 1 activity were neutralized with rabbit antiserum to human IL 1 and also stimulated the growth ([3H]thymidine incorporation) of long-term fibroblast-like cell lines derived from human synovial rheumatoid exudate. Two other crystals associated with articular inflammation were tested: hydroxyapatite was a much less potent stimulus compared with MSU crystals, and calcium pyrophosphate dihydrate did not stimulate IL 1 release from human monocytes or synovial fluid MNC. As a model for the inflammatory consequences of acute and chronic overproduction of IL 1, gout is the only sterile inflammatory disease where the local and systemic pathology is compatible with such overproduction; raised IL 1 levels have been found at the site of inflammation, and a necessary etiologic agent, crystalline urate, has been shown unequivocally to be a direct activator of mononuclear IL 1 release.