The investigation was carried out in order to clarify the reason for the reduction in nutritive value of browned protein, by using casein as a model protein. In a previous report, we found ε-N-deoxyfructosyl-lysine as an absorption delayed material in the small intestinal content of rats using browned 14C-lysine labeled casein which was prepared under a mild condition (37°C and R.H. 70%). This study was performed to elucidate the change in amino acid composition of casein browned by amino-carbonyl reaction with glucose under the mild condition, and to determine the protein efficiency ratio (PER) and digestibility of browned casein with or without supplementary lysine. As a result, PER of browned casein was decreased, but it was recovered to the control level by addition of the equal amount of lysine to that lost by browning reaction. Therefore, the limiting amino acid of browned casein was assumed to be lysine. As the true digestibility of browned casein was not decreased, it was suggested that the reduction in its nutritive value was mainly due to the conversion of lysine in browned casein to ε-N-deoxyfructosyl-lysine, which remains in the small intestine as an absorption delayed material.