Fragments of synovium from patients with rheumatoid arthritis survive in defined tissue culture medium in the absence of added serum and, after 3-4 days, release into the medium enzyme capable of degrading undenatured collagen. Maximal activity is observed at pH 7-9 but the enzyme is inactive at pH 5. At temperatures of 20 degrees and 27 degrees C, collagen molecules in solution are cleaved into 3/4 and 1/4 length fragments with minimal loss of negative optical rotation, but with loss in specific viscosity of approximately 60%. Above 30 degrees C the fragments begin to denature and denaturation is complete at 37 degrees C. If the enzyme is not inhibited at this stage the large fragments are broken down further to polypeptides of low molecular weight. Reconstituted collagen fibrils and native fibers at 37 degrees C are cleaved to the low molecular weight fragments, although the fibrils are resistant to breakdown at lower temperatures (20 degrees -27 degrees C). It is proposed that the production of such an enzyme by inflamed and proliferating rheumatoid synovium may be responsible for some of the destruction of collagenous structures that accompanies rheumatoid arthritis.