Thioglycollate-stimulated mouse macrophages release a specific collagenase into their medium during in vitro culture. The macrophage collagenase has been characterized as a typical metal proteinase which catalyzes the cleavage of the native collagen molecule into three and one-quarter fragments. The extracellular accumulation and low activity in cell lysates suggest that collagenase is a secretion product of the stimulated macrophage. Prolonged secretion of the enzyme at a constant rate for more than 7 days in culture and its inhibition by cycloheximide provide evidence for biosynthesis in vitro. In contrast, secretion of collagenase is barely detectable from unstimulated macrophages which can, however, be stimulated to secret the enzyme by ingestion and intralysosomal storage of latex particles or dextran sulfate. Macrophages laden with latex, an undigestable particle, continue to release collagenase for at least 20 days. Several established mouse cell lines have also been examined for their capacity to secrete collagenase. Collagenase is one of a class of inducible neutral proteinases by which the activated macrophage can modify its extracellular environment.