A Mr 95,000 matrix metalloproteinase (MMP) produced by rat mammary carcinoma cells has been isolated and characterized. The MMP was secreted in a proteolytically inactive form that was free from bound tissue inhibitor of metalloproteinases. The enzyme was highly glycosylated as evident from an apparent drop of Mr from 95,000 to 83,000 after treatment with N-glycanase. Rotary shadowing electron micrographs of purified proenzyme preparations revealed a uniform set of ellipsoidal molecules. Treatment of the proenzyme with 1% SDS resulted in generation of catalytic activity and exposed a cryptic unpaired Cys residue. The latent proenzyme may be activated in at least three additional ways: either spontaneously upon storage, by treatment with organomercurials, or by limited proteolysis by trypsin. Each mode of activation yielded a distinct pattern of cleavage of the enzyme. The activated enzyme cleaved gelatin (denatured type I collagen) and native type IV and V collagen at 30-37 degrees C. Noncollagenous proteins including alpha 1-proteinase inhibitor, casein, and fibrinogen also were cleaved. The rat mammary carcinoma cell line that produces the Mr 95,000 MMP is composed of two distinct (epithelial- and myoepithelial-like) cell types. The enzyme is expressed constitutively by the epithelial cells. This suggests that expression of the Mr 95,000 MMP is regulated differently from that of interstitial collagenase, which is produced by the epithelial cells only in response to specific inductive factor(s) from the myoepithelial-like cells. Monoclonal antibodies raised against the purified latent Mr 95,000 form of the enzyme bind specifically to the Mr 95,000 MMP and have been used to localize the enzyme to the Golgi region and cytoplasmic granules of the epithelial cells.