Synovial tissue from patients with rheumatoid arthritis produces lysis of gels of reconstituted collagen fibrils in culture and releases soluble collagenase when cultured in collagen-free medium. Collagen molecules in solution at neutral pH at 20° and 27°C are cleaved by the synovial enzyme into ¾ and ¼ length fragments. In this respect the action of synovial enzyme is similar to that of amphibian collagenase and distinct from that of bacterial collagenase. At 37°C reconstituted collagen fibrils and native fibers are attacked by the enzyme and further degraded to polypeptides of low molecular weight. These polypeptides are produced only after denaturation of the larger fragments, which occurs at temperatures near 37°C.