Specific activity of superoxide dismutase (SOD) in cataractous lenses was examined. Immunoprecipitation with monospecific antibodies showed an accumulation of catalytically inactive, but antigenically reactive enzyme molecules in lenses with various types of cataracts. The inactive enzyme molecules can be demonstrated in all kinds of cataracts, including dense, mature cataracts with very low or nondetectable enzyme activity. In cataracts from diabetic patients, the mean enzyme activity was higher than in nondiabetic patients. This was especially marked in nuclear cataracts, in which the specific activity of SOD was higher than in similar cataracts from nondiabetics.