Collagenase occurs in animal tissues in at least two molecular forms; free enzyme and collagen-bound enzyme. This study was undertaken to investigate which form is principal during the postpartum collagen degradation. Ethylenediaminetetraacetic acid, which depresses the calcium concentration in the serum, did not affect the postpartum collagen degradation. Histological study shows that the serum containing inhibitor such as alpha 2-macroglobulin was widely scattered within the endometrium on the day of parturition. Since the calcium is necessary for collagenase to bind collagen and that the collagen-bound collagenase together with or without alpha 1-macroglobulin is activated to a same level by protease, these results suggested the principal molecular form of collagenase during the postpartum involution is collagen-bound enzyme.