Aging of the normal human lens is accompanied by oxidation of protein sulfhydryl groups to disulfide groups. Although this has been known for many years, very little is known about the exact amino acid residues involved. In addition, almost nothing is known concerning the temporal sequence of this oxidative process over the lifetime of the individual. To address these two concerns for alpha-A crystallin, the polypeptide was purified from total proteins of the human lens, followed by digestion with lys-C endoprotease. Mass spectral analysis of the resulting fragments demonstrated that the two cysteine residues (cysteine-131 and cysteine-142) are present as a mixture of an intramolecular disulfide bond and free sulfhydryl groups. Reverse phase chromatography was used to resolve and quantitate the relative amounts of the two forms present in alpha-A crystallin from normal lenses of different age. Even in very young lenses (4 months and 5 months of age) there is significant oxidation of the two cysteine residues. However, the oxidative state of these two residues does not significantly change during the next approximately 27 years of age, after which there is an increase in the relative amount of intramolecular disulfide bonding. Together, these results have identified and quantitated the relative change in the oxidative state of two specific cysteine residues of alpha-A crystallin in human lenses of different age, and have established that age-dependent oxidation of these two residues occurs primarily during the later part of life.