Evidence is presented for the synthesis of Interleukin-1 (IL-1) by joint synovial tissue and chondrocytes. Purified preparations of mouse and human recombinant forms of this factor stimulate the synthesis of a secretory protease by cartilage. The IL-1 stimulated chondrocyte protease is capable of converting latent collagenase to its active form. Other proteases such as trypsin and the mercurial aminophenyl mercuric acetate will not activate collagenase in the absence of this protease. Evidence is presented showing that chondrocytes synthesize IL-1 suggesting autocrine control of cartilage matrix turnover.