Individual crystallins, urea-soluble and urea-insoluble proteins were isolated from the nucleus and cortex of types I-IV cataractous lenses and normal lenses. The levels of protein sulphydryis (P-SH), disulphides (S-S), as well as surface (F-SH) and buried (S-SH) in these proteins were determined by reaction with 5, 5′-dithiotris-(2-nitrobenzoic acid) or performic acid oxidation followed by amino acid analysis.During nuclear colour development there is a progressive decrease in the sulphydryl content of the crystallins. In the nuclei of advanced cataractous lenses, the P-SH decreases to 10% of the levels found in the normal nucleus. Similar but smaller changes take place in the cortex. No specific changes were found between the crystallins, with the exception ofβs crystallin.The cysteine remains constant in all lens types suggesting no higher oxidation products are formed.There is a significant shift in the distribution of cysteine in the nucleus of type III and IV lenses. Urea-insoluble proteins are the predominant species, accounting for about 70% of the total cysteine pool. This is consistent with the accumulation of modified insoluble polypeptides during senile nuclear cataract formation.