Collagenase activity was found in the corneal epithelium of ten cases of corneal diseases associated with either acute or chronic corneal ulcerations. Sixteen cases, which included autopsy corneas, nondiseased corneas, and diseased corneas wihout ulcerations failed to reveal detectable activity. The collagenolytic activity was demonstrated by lysis of reconstituted collagen gels upon which epithelial explants were cultured in vitro. The enzyme was active at physiological pH, and inhibited by disodium ethylenediaminetetraacetic acid (EDTA). The role of collagenase in the pathogenesis of corneal ulcerations is discussed.