SummaryThe amounts of protein extractable from wool following oxidation with performic acid, and the yields of high. sulphur (y·keratose) and low· sulphur (",-keratose) fractions have been shown to depend markedly on the conditions of dialysis used to remove excess performic acid reagent. The results are explained by assuming that denaturation occurs in warm dilute formic acid. Following denaturation in this way, or following exposure to solutions of trichloroacetic acid or to alkali at pH 11, the extracted proteins are more readily separated into high. and low· sulphur fractions. It is postulated that contaminant high. sulphur protein is responsible, in part, for the chromatographic heterogeneity and for the variation in amino acid composition of separated low· sulphur proteins. Binding of this high· sulphur contaminant protein fraction to the low·sulphur proteins is shown to be by secondary valence bonds. The percentage of high. sulphur proteins in wool is higher than the estimates of earlier workers.