Casein and egg albumin were reacted with nitrite at pH 3. The reaction mixtures containing nitrite yellowed when the reaction began; after 24 hours of reaction with 7.5 X 10(-3) M nitrite, about 70% of added casein was sedimented as yellow precipitate. There was a considerable loss of lysine, arginine, tyrosine, and tryptophan residues. This was determined from the amino acid compositions of the proteins obtained by the reaction with 2.5 X 10(-2) M nitrite at pH 3 for 48 hours. An amino acid elution pattern of the treated casein showed five peaks due to new amino acids; three of these were provisionally identified to be cysteic acid, methionine sulfone, and 3-nitrotyrosine.