Analytical methods were adapted to the determination of methionine sulfoxide and methionine sulfone in casein. Direct determination by alkaline hydrolysis appears to be more accurate than indirect determination by the carboxymethylation‐performic oxidation method. Proteolysis of casein with Pronase was adopted as an in vitro digestibility test. In the conditions chosen, 41% of the methionine residues of casein were released as free methionine. A solution of casein was treated with up to 0.12M hydrogen peroxide. No methionine sulfone was formed. Most of the methionine residues were oxidized into methionine sulfoxide. When milk was treated with 0.018M hydrogen peroxide, 51% of its methionine residues was transformed into methionine sulfoxide. Hydrogen peroxide‐oxidized casein samples were submitted to the Pronase digestibility test. Release of free methionine was inversely related to the methionine sulfoxide content. No free methionine sulfoxide was released from casein. Release of other amino acids was little affected by hydrogen peroxide treatment. Similar results were found with m