1. The influence of temperature, moisture content and the presence of glucose on the level of available methionine and cysteine in casein was studied. 2. Differences between total and available methionine and cysteine contents of heated casein (90 degrees for 24 h) were determined by an in vitro method. The maximum losses in total and available methionine content were 22 and 51% respectively. The losses in total and available cysteine content were 24 and 100% respectively. 3. The results indicated that for heated casein the release of amino acids by proteolytic enzymes was less complete than for native casein. 4. The results of rat growth assays suggested that diets containing oxidized casein are less well utilized by rats than those containing native casein. The decrease in body-weight of rats receiving the diets containing oxidized casein could be counteracted by the addition of methionine and 20 g unoxidized casein/kg diet. 5. There was a lower level of some available amino acids (determined after enzymic hydrolysis using pancreatopeptidase E (EC 3-4-4-7), leucine aminopeptidase (EC 3-4-I-I) and prolidase (EC 3-4-3-7)), including those essential for the rat, in oxidized casein as compared with native casein. 6. Cysteic acid, in oxidized casein, probably makes impossible the utilization of the amino acids in its neighbourhood. 7. From the differences in the available amino acid contents of the native, oxidized and heated casein it was concluded that the oxidation of casein causes the formation of complexes in the polypeptide chain, resistant to enzymic hydrolysis, but to a much lesser extent than does heating.