Procollagenase M(r) 85,000 (SDS-PAGE) was purified from buffy coat to homogeneity and represents a stable single polypeptide chain forming the entire proenzyme. The procollagenase can be activated by various proteinases, e.g. trypsin, chymotrypsin, cathepsin G, kallikrein and stromelysin and by different mercurial compounds. Proteolytic conversion of the latent enzyme to the active form by chymotrypsin is accompanied by a molecular weight reduction to an apparent M(r) 64,000. This active enzyme lacks the first 79 N-terminal residues. Activation by trypsin leads to a latent intermediate of apparent M(r) 70,000, lacking 48 N-terminal residues. The active enzyme is therefore generated upon prolonged incubation with trypsin by further cleavage of 22 N-terminal residues. Another latent intermediate form with apparent M(r) 69,000 is generated from the proenzyme upon incubation with leukocyte elastase by N-terminal cleavage of 53 or 64 residues, respectively. However, latent collagenase cannot be activated by plasmin. Activation by different mercurial compounds finally results in the formation of active collagenase with apparent M(r) 64,000. In contrast to the proenzyme, active collagenase can autolyse to give active M(r) 57,000 and 45,000 intermediates and two M(r) 28,000 fragments. Purification of latent leukocyte gelatinase yields three final products with apparent M(r) 98,000, 125,000 and 220,000 (SDS-PAGE; non reduced). Upon reduction, only the M(r) 98,000 form can be detected. The latent gelatinase can be activated in a similar manner as collagenase. Proteolytic activation by trypsin leads after N-terminal cleavage to an active gelatinase with sequence homology to leukocyte collagenase.