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Collagenase activity was measured by direct assay in skins from 12 patients afflicted with systemic sclerosis. In seven of those cases where extensive involvement of the forearm and trunk skin existed, collagenase activity of the involved skin was minimal or absent. Moreover, in the same patient, regions of marked skin involvement (e.g., forearm) showed no collagenase activity, when clinically uninvolved areas (thigh) exhibited normal or nearly normal levels of enzyme activity. In other patients where clinical symptoms were systemic and not associated significantly with the skin, collagenase activity approximated normal levels. Measurements of collagenase activity and tensile strength in another condition (basal cell carcinoma) that includes changes in mechanical properties of skin that any be regarded as the opposite end of the spectrum from those of sclerodermatous skin support a general correlation between collagenase activity and tensile strength. These studies indicate that the major defect responsible for the hidebound skin lesions of scleroderma may be decreased collagenase activity.