Calpains (calcium-dependent cysteine proteinases; optimum pH 7.0-7.5) have been regarded as intracellular proteinases. We examined the cell-free components of synovial fluid from 14 patients with osteoarthritis and demonstrated the existence of calpains, as the caseinolytic activities of chromatographic fractions, together with calpastatin, the specific endogenous inhibitor of calpains. The presence of these calpains and calpastatin was verified by immunoblotting with their respective specific antibodies. Calpain fractions showed proteoglycan-degrading activity. The results suggest that the calpain-calpastatin system may contribute to the turnover of cartilage matrix components.