Beta B2 crystallin has been prepared from total protein of normal (transparent) and cataractous human lenses. After digestion with endoprotease lys-C, the crude digests were resolved on a C18 reverse phase column. The lys-C digests of beta B2 crystallin from cataractous lenses consistently showed the presence of a major peptide that was not present in the lys-C digests from normal lenses. Treatment of this peptide with dithiothreitol resulted in the production of two peptides, corresponding to beta B2 crystallin sequences 18-41 and 48-67, containing cysteine-37 and cysteine-66, respectively. The results demonstrated that intramolecular disulfide bonding of these two residues had occurred in vivo. Based upon previous knowledge of the three dimensional structure of beta B2 crystallin, formation of this disulfide bond suggests that significant denaturation of the beta B2 crystallin molecule has occurred during the process of human lens opacification.