High levels of collagenase are present in cervical extracts and in the circulation in women at parturition. This study examines the content of collagenase in human placentas at term, the possible contribution to circulating enzyme, and the changes that occur at parturition. Active and latent forms of collagenase are detectable in placentas with apparent relative molecular mass of 60,000 and 65,000 d, respectively. Gel-filtration chromatography was used to identify the presence of excess tissue inhibitor of metalloproteinase as the major collagenase inhibitor in the extracellular matrix of human placentas. After the onset of labor, there was a significant increase in total collagenase activity. Inactivation of the tissue inhibitor of metalloproteinase by reduction of placental extracts with dithiothreitol and alkylation with iodoacetamide resulted in a twelvefold to seventeenfold increase in collagenase activity. Umbilical cord collagenase levels were significantly lower than those in maternal circulation. The possibility of circulating collagenase originating from placenta in labor is discussed.