In order to heighten the nutritional value of food, the lysine-rich food has been made. However, added lysine reacts on sugar in food and results in loss of lysine, browning of food arid further bitterness. The condition which causes the loss of lysine and the browning has been examined and the method for preventing it has been studied. 1. Most of amino acids react on sugar and result in the browning, especially in case of addition of lysine it is remarkable. It has been considered that this is due to the fact that lysine has a reactive e-amino group. The conditions of lysine-sugar reaction were as follows: a) Effect of the kind of sugar: Pentose reacted most strongly on lysine and hexose and disaccharide followed. b) Effect of the concentration of sugar: As the concentration of sugar increased, the browning became stronger. c) Effect of pH: The loss of lysine and the browning were weak in acidic solution, but strong in alkaline. d) Effect of buffer solution: According to the kind of buffer solution used and its concentration, the loss of lysine was different. In phosphate buffer solution, the reaction was accelerated but in Tris buffer solution, it was not. e) Effect of heat treatment : In heating up to 100℃, the loss of lysine was not so strong, but over 100℃ became strong. f) Effect of other kinds of amino acids: Cystine, glutamic acid and histidine inhibited the lysine-sugar reaction, but tryptophan and tyrosine accelerated it. g) Effect of metal : In the existence of Fe^+++ion, the reaction was accelerated, but Ca^++ and Mg^++ion inhibited the reaction. 2. Lysine reacted strongly on not only sugar, but also aldehyde which produced when treating food and resulted in the browning. 3. In order to prevent the loss of lysine and the browning, several surfactants which seemed to give any effect on the amino group of lysine were added to the solution of lysine and sugar, which, afterwards, were heated, but the loss of lysine could not be prevented. 4. The decrease of lysine and the browning on the liquid food added lysine were examined. Adding lysine to fermented milk or orange juice and stored at 5℃, the browning has hardly caused, but at 30℃ for more than one month, the decrease of lysine and the browning of liquid have been observed. 5. On the lysine-rich bread made by the usual method which lysine was added to dough with water, 20 to 30 per cent of lysine added was lost and adding 0.3 per cent of lysine to bread, the browning caused up to such a grade that it could not be sold in market. Therefore, the manufacturing method for lysine-rich bread which does not cause the browning has been studied. So far, lysine was dissolved in water and then it was added to dough, however, adding lysine to dough after having suspended it in shortening oil, little browning has been observed. Further, adding it to dough after having coated lysine with hard oil, the browning has been prevented. 6. The manufacturing method for lysine-rich food adding ε-N-acetyl-L-lysine instead of lysine has been found, and the browning has been perfectly prevented by this method.