Attempts were made successfully to perform the amidation of carboxyl groups of lysozyme in liquid ammonia. Prior to the chemical modification, stability of lysozyme in liquid ammonia was investigated under various experimental conditions. As the results, it was found that there are no significant effects of liquid ammonia on the physical as well as chemical and enzymatical properties of lysozyme when lysozyme was dissolved in liquid ammonia below -45℃. Amidation of carboxyl groups of lysozyme was achieved by the ammonolysis of the active forms of ester. The esterification of carboxyl groups of lysozyme was carried out by three diffetent methods pmploying the following reagents: (1) methanol with hydrochloric acid, (2) carbodiimide, (3) Woodward's reagent. In the case of methanol-HCl, esterification was performed in aqueous solution and esterified lysozyme was ammonolyzed in liquid ammonia resulting in amidized lysozyme. In this method carboxyl group could be amidized without any denaturation when it was carried out at low temperature below -45℃, though the reaction proceeded so slowly. Denaturation occurred accompanying insolubilization of lysozyme at room temperature. In the case of carbodiimide, two methods were employed. Carboxyl groups of lysozyme were modified with 1-cyclohexyl-3-(2-morphinoethyl) carbodiimide (CMC) in aqueous solution followed by successive ammonolysis of CMC-lysozyme to amidized lysozyme in liquid ammonia. On the other hand, lysozyme and CMC were simultaneously dissolved in liquid ammonia below -45℃, and the reaction was completed producing amidized lysozyme. In this method the chemical modification occurred selectively in comparison with that in aqueous solution, and six carboxyl groups were amidized. The remaining activity of product was measured to be about 70 per cent. In the case of Woodward's reagent, N-ethyl-5-phenylisoxazoline-3-sulfonate (WR), the reagent and lysozyme were reacted in liquid ammonia, and yielded specifically amidized lysozyme of which three carboxyl groups were modified and remaining activity was about 75 per cent. Thus, a new method of the chemical modification of carboxyl groups of lysozyme was established by using liquid afnmonia as solvent as well as chemical reagent. As the modification with CMC or WR in liquid ammonia are particularly selective, this method seems to be available for the study of the relationship between structure and function of lysozyme.