This work is a study of the influence of individuality, regime and age of the cow on the susceptibility of milk to the heat liberation of sulfhydryls groups. It has been shown that the titrable —SH content of milk varies with the individuality of the cow but not with the age. In the case of the oldest cows only. the sulfhydryls content of milk decreased when the change was made from winter feed to summer pasture. These observations suggest the hypothesis of the important influence of heredity. The liberation of sulfhydryls by heating at 80C during 30min. is more consistently, influenced by the regime, being more pronounced in winter than in summer. It is indicated that some of the changes which are known to occur in milk when cows are transferred from winter regime to summer pasture are probably responsible for this increased resistance of milk to sulfhydryls liberation by heat.