Synovial fluid basic calcium phosphate (BCP) crystals are associated with severe destructive arthropathies that are characterized by synovial proliferation and digestion of articular collagenous structures. BCP crystals are potent mitogens, which may account for this proliferation. The role of collagenase in articular degradation is controversial because, despite the massive loss of collagen, no studies have confirmed collagenolytic activity in synovial fluid, as originally reported. We investigated collagenase messenger RNA induction and enzyme activity in human foreskin fibroblasts proliferating in response to stimulation with BCP crystals, and analyzed the associated secreted proteins. Northern blots revealed a dose-dependent accumulation of collagenase message, evident by 4 hours and continuing to at least 36 hours, in BCP-stimulated cultures. One- and 2-dimensional polyacrylamide gel electrophoresis of conditioned media from BCP crystal-stimulated cultures revealed the selective induction of 2 proteins with molecular weight and pI values consistent with those of collagenase. Increased enzyme activity was also found. Thus, the mitogenic response of fibroblasts to BCP crystals is accompanied by collagenase induction and secretion, supporting the hypothesis that they act as a mediator of joint destruction in BCP crystal-associated arthropathies.