The uterus of the rat contains a small metalloproteinase that digests Azocoll and proteoglycan. The activity of this enzyme is elevated in the postpartum uterus and parallels the rate of breakdown of matrix proteoglycan (Sellers, A. and Woessner, J.F., Jr., Biochem. J. 189: 521, 1980). The enzyme has been purified to homogeneity. Its molecular weight is 28,000 for the latent form of the enzyme and 19,000 for the active form, as determined by SDS/PAGE. The enzyme has no action on collagens of type I, III, IV or V, but it does digest gelatins of these 4 types. Digestion of type I gelatin is most pronounced for the alpha-2 chain, which is cleaved to two major bands. The B chain of insulin is cleaved at Ala14-Leu15 and Tyr16-Leu17. Proteoglycan is degraded, but no action can be detected against elastin. Both zinc and calcium ions are required for activity. Low levels of phosphoramidon or Zincov are not inhibitory. Detailed comparisons with human gelatinase (matrix metalloproteinase 2) and stromelysin (matrix metalloproteinase 3) show that the uterine proteinase has a distinctive pattern of specificity. The properties match those of human Pump-1 as reported by Quantin et al., Biochemistry 28: 5327, 1989. It is proposed to designate this proteinase as matrix metalloproteinase 7.