Late in 1951, Pearse (18) reported that after peracetic or performic acid oxidation human and rat hair cortex’ gave a blue coloration with the AshbelSeligman carbonyl method and a red color with Schiff reagent. He recorded that the positive performic acid Scuff reaction was not prevented by fixation in boiling chloroform-methanol, though that of myelin was. He therefore excluded fat as a source of the reaction. In support of this view he reported that the Nadi indophenol synthesis, which according to Lison (17) may indicate the presence of fat peroxides, is not effected by hair cortex after performic acid oxidation. In further support of this view, he reported that while dimedone (saturated in 5 % acetic alcohol) and 2 % phenylhydrazine (in the same solvent) each reversed the performic acid induced positive Schiff reaction of erythrocytes, they had less effect on that of hair cortex. A 24 hour treatment with phenylhydrazine at 60#{176}C. greatly weakened the Schiff reaction of hair cortex, but so did the acetic alcohol which he used as a solvent for phenylhydrazine. By these tests, Pearse excluded aldehyde as the substance reacting with the Schiff reagent. Mercuric chloride and iodine treatments (before performic acid) did not interfere, but no other tests for -SH were made. Bromination was not done. The positive Schiff reaction was attributed to a-aminopropionic acid-fl-sulfinic acid [HOOC-CH(NH,)-CHr--SO2H], hereafter called cystine sulfinic acid. The concurrently present intense basophilia toward methylene blue, which is produced in hair cortex by performic (and peracetic) acid oxidation, he assigns to the cysteic acid [HOOC-CH(NH2)-CH,---SO,H] residues which are known to be produced in hair keratin by peracetic and performic acid oxidations. We fully agree with this interpretation of the intense (metachromatic) basophilia of hair cortex after cystine disulfide cleavage reactions with oxidants (14). Inasmuch as our own studies on other tissues (10,16) had indicated that the Schiff reactionensuing afterperformic or peracetic acid oxidation was usually to be assigned to aldehyde cleavageof peroxides derived from carbon-carbon double