Collagenases produced by human polymorphonuclear leukocytes, human lung fibroblasts, and rabbit pulmonary alveolar macrophages were compared in their ability to digest soluble native type I and type III collagens. While the fibroblast and macrophage collagenases attacked the two substrates at approximately equal rates, the leukocyte collagenase attacked type I collagen preferentially (15:1) in comparison to type III collagen. This was true with human or rabbit collagen substrates. Thus, proteolysis of collagen, particularly in acute inflammation, may have a significant role in controlling the types of collagen present in connective tissue.