Young male rats were used in five experiments to study the utilization for growth of methionine sulphoxide, and the relationship between the sulphoxide content in the diet and the level of microbiologically determined methionine activity in blood or blood plasma. In one nitrogen-balance experiment methionine and methionine sulphoxide were compared as supplements to a casein diet and a fish-meal diet. 2. Methionine sulphoxide was poorly utilized for growth when testd as the sole sulphur amino acid in an amino acid diet. Substitution of one-third of the sulphoxide with cystine improved utilization so that it approached that of methionine. 3. Methionine alone and in combination with methionine sulphoxide were added to a soya-bean-meal diet. The sulphoxide showed no adverse effect on growth. 4. Fish meal in which methionine had been oxidized to methionine sulphoxide was tested alone and in combinations with unoxidized fish meal. Only when the oxidized meal was given alone was there an appreciable effect on growth. The fish meal used were low in cystine. 5. Whereas both methionine and methionine sulphoxide improved the N balance when a casein diet was given, there was no effect when a fish-meal diet was given. 6. There was a linear relationship between methionine sulphoxide content in the amino acid diets and the methionine activity in the blood plasma. Methionine sulphoxide added to a soya-bean-meal diet or present in oxidized fish meal gave a curvilinear relationship, and the observed activities were lower than with the amino acid diets. Methionine activity in blood could not be used as an indicator of moderate amounts of methionine sulphoxide in protein-containing diets.
