Human rheumatoid synovial cells in culture secrete at least three related metalloproteinases that digest extracellular matrix macromolecules. One of them, termed matrix metalloproteinase 2 (MMP‐2), has been purified as an inactive zymogen (proMMP‐2). The final product is homogeneous on SDS/PAGE with M_r= 72000 under reducing conditions. The NH₂‐terminal sequence of proMMP‐2 is Ala‐Pro‐Ser‐Pro‐Ile‐Ile‐Lys‐Phe‐Pro‐Gly‐Asp‐Val‐Ala‐Pro‐Lys‐Thr, which is identical to that of the so‐called ‘72‐kDa type IV collagenase/gelatinase’. The zymogen can be rapidly activated by 4‐aminophenylmercuric acetate to an active form of MMP‐2 with M_r= 67000, and the new NH₂‐terminal generated is Tyr‐Asn‐Phe‐Phe‐Pro‐Arg‐Lys‐Pro‐Lys‐Trp‐Asp‐Lys‐Asn‐Gln‐Ile. However, following 4‐aminophenylmercuric acetate activation, MMP‐2 is gradually inactivated by autolysis. Nine endopeptidases (trypsin, chymotrypsin, plasmin, plasma kallikrein, thrombin, neutrophil elastase, cathepsin G, matrix metalloproteinase 3, and thermolysin) were tested for their abilities to activate proMMP‐2, but none had this ability. This contrasts with the proteolytic activation of proMMP‐1 (procollagenase) and proMMP‐3 (prostromelysin). The optimal activity of MMP‐2 against azocoll is around pH 8.5, but about 50% of activity is retained at pH 6.5. Enzymic activity is inhibited by EDTA, 1,10‐phenanthroline or tissue inhibitor of metalloproteinases, but not by inhibitors of serine, cysteine or aspartic proteinases. MMP‐2 digests gelatin, fibronectin, laminin, and collagen type V, and to a lesser extent type IV collagen, cartilage proteoglycan and elastin. Comparative studies on digestion of collagen types IV and V by MMP‐2 and MMP‐3 (stromelysin) indicate that MMP‐3 degrades type IV collagen more readily than MMP‐2, while MMP‐2 digests type V collagen effectively. Biosynthetic studies of MMPs using cultured human rheumatoid synovial fibroblasts indicated that the production of both proMMP‐1 and proMMP‐3 is negligible but it is greatly enhanced by the treatment with rabbit‐macrophage‐conditioned medium, whereas the synthesis of proMMP‐2 is constitutively expressed by these cells and is not significantly affected by the treatment. This suggests that the physiological and/or pathological role of MMP‐2 and its site of action may be different from those of MMP‐1 and MMP‐3.