The specific activity of the enzyme glyceraldehyde 3-phosphate dehydrogenase declines as a function of age. Immunotitration with monospecific antibodies demonstrated that with age there is an increase of catalytically defective, but antigenically reactive enzyme molecules in the lens. Antiserum, produced against denatured enzyme, removed the inactive molecules from lens homogenates, without effects on the levels of enzyme activity. These studies suggest that inactive enzyme molecules in aging lenses are totally devoid of catalytic activity, and are at least partially denatured.