The occurrence of cysteine in the normal crystalline lens has been suspected since the work of Arnold1 in 1910. Arnold considered the positive color reaction obtained with sodium nitroprusside as proof of the presence of cysteine. Jess,2 in 1922, found that cysteine and its oxidized form, cystine, make up 2.3 per cent of alpha crystallin and 4.9 per cent of beta crystallin, the soluble proteins of the lens, and are absent from the insoluble protein residue. In 1922, von Reis3 found that the reaction to sodium nitroprusside, which is characteristic of cysteine-containing substances and is positive in normal lenses, was decreased or entirely negative in the cataractous lens. It was absent only in the nuclei of immature senile cataracts, while in mature cataracts it was negative except in the thin layer of cortex next to the capsule. Jess4 confirmed his findings and obtained similar results in