Free epsilon-amino groups in soluble and insoluble proteins were measured in clear human lenses and in diabetic and nondiabetic senile cataractous lenses. The free epsilon-amino group content of soluble and insoluble proteins was significantly lower in diabetic cataracts than in clear lenses and nondiabetic senile cataracts. Our results seem to demonstrate that nonenzymatic glycosylation of lens proteins could play a role in the pathogenesis of diabetic cataract.