Changes in activities of ascorbate free radical (AFR) reductase (NADH:AFR oxidoreductase) and H2O2-dependent NADH oxidation were correlated with levels of insoluble protein in senile cataractous human lenses. The H2O2-dependent NADH oxidation activity was measured to reflect the content of free glutathione. AFR reductase activities in all the cataractous lenses assayed here tended to decrease with increase of insoluble protein contents. A similar tendency in the relationship between lens protein aggregation and H2O2-dependent NADH oxidation activities, i.e. free glutathione contents was recognized in the lenses with pale yellow, yellow or dark yellow nucleus. However, for the highest levels of insoluble protein, some of the brunescent cataractous lenses exhibited very high activities of H2O2-dependent NADH oxidation, and some brunescent lenses had very low activities. From the above results, it is suggested that lens protein aggregates in the brunescent and non-brunescent cataractous lenses may be formed through significantly different oxidation processes, respectively. The possible mechanisms such as free radical reaction and disulfide bond formation are discussed.