Stromelysin-1 was purified from human articular cartilage and compared to synovial fibroblast enzyme and to recombinant enzyme. If the latent enzyme was incubated at pH 5.5 with substrates such as aggrecan, it spontaneously became active. Incubation of latent zymogen alone at pH 5.5 gave increasing activation over a period of at least 5 hours. However, this activation process was not accompanied by any shift in molecular weight even after continued incubation for 18 hours. Maximum activity observed was 45-60% of that seen with APMA activation. Stromelysin-1 has a pH optimum of 5.5-6.5 on various macromolecular and peptide substrates. Interaction with TIMP is reduced at pH 5.5 relative to that at 7.5. The hypothesis is presented that osteoarthritis may be initiated by acid activation of stromelysin-1.