Collagenase has been purified from the culture medium of a human myometrial smooth muscle cell line, and the properties of the pure enzyme compared to those of collagenase from another human mesenchymal cell, the fibroblast. The smooth muscle collagenase was purified using a new, rapid, and convenient three-step purification procedure consisting of chromatography on iminodiacetate-agarose chelated with zinc and on Cibacron Blue-agarose followed by gel filtration on Ultrogel AcA-44. The resultant pure collagenase is secreted as a zymogen indistinguishable from that of the fibroblast enzyme in molecular weight, amino acid composition, and in the nature of its conversion to active enzyme by trypsin. The amino acid sequence of the two enzymes at the trypsin cleavage site is the same. The two collagenases are also indistinguishable immunologically and display essentially identical kinetic behavior on a variety of collagen substrates. Although the two collagenases appear to be identical proteins, the mechanisms which regulate their production appear to be very different. Glucocorticosteroids, which inhibit collagenase production in human skin fibroblasts are without effect in the uterine smooth muscle cell. In contrast, the smooth muscle cell appears to require a component present in fetal bovine serum in order to produce the enzyme.