Collagenase is a highly specific neutral protease which acts by cleaving the collagen molecule into fragments, at a site three-quarters of the distance from the amino terminus. Collagenase has been assumed to be connected with the destruction of the pathological connective tissue which accompanies inflammatory conditions such as rheumatoid arthritis and interstitial lung disease. Despite the association of eosinophils with wound-healing and fibrotic processes, their collagenolytic ability has been poorly defined and it was not until 1984 that human eosinophils were shown to contain an enzymatic activity which degrades collagen. Eosinophils contain a metalloprotein that degrades types I and III collagens and no collagenolytic activity against types IV and V collagens is detected. Some experiments suggested that the eosinophil collagenase was different from neutrophil collagenase which cleaves type I collagen preferentially. Eosinophils may play a role in the alterations in connective-tissue matrices seen in physiological and pathological states.