A neutral metalloproteinase has been isolated and purified from adherent rheumatoid synovial cells in culture. This protease, named matrix metalloproteinase 3, (MMP-3) degrades gelatin, proteoglycan, fibronectin, type IV collagen, laminin, and the N propeptide of type I procollagen. It can be separated from MMP-2 (a potent gelatinase), and MMP-1, an interstitial collagenase. MMP-3 is released from cells as a proenzyme of 55 Kda. Activation by trypsin or organic mercurials produces 2 active species of 45 Kda and 28 Kda. The enzyme contains zinc as an intrinsic component and requires calcium for conformational stability. In concert, active MMP-1, -2, and -3 can destroy all significant structural proteins of joint structures.