In inflamed tissue sites characterized by on-going matrix degradation, the matrix metalloproteinases are secreted as latent precursors which are capable of proteolysis only after extracellular activation. Such areas often contain locally increased numbers of mast cells capable of releasing complexes between heparin proteoglycans and fully active endopeptidases with either tryptic (tryptase) or both tryptic and chymotryptic (chymase) activity. We have examined the ability of purified human skin chymase to activate human interstitial procollagenase (matrix metalloproteinase-1) in the absence and presence of heparin, the physiologic associate of chymase. Our studies indicate that chymase activates procollagenase in a time- and concentration-dependent manner. Heparin was found to increase markedly the rate at which chymase activates procollagenase both by accelerating the cleavage of procollagenase and also by preventing its further degradation. Moreover, we found that chymase activates procollagenase directly by cleaving the Leu83-Thr84 bond, without formation of any intermediate species. This is a novel mechanism for procollagenase activation, which contrasts sharply with the activation mechanisms of other activators studied so far. The ability of chymase to activate procollagenase suggests that chymase plays an active role in matrix degradation at tissue sites where mast cells coexist with extracellular procollagenase.