PROBLEM TO BE SOLVED: To provide a method for selectively acylating an amino group in a peptide or protein which has two or more reactive nucleophilic functional groups.SOLUTION: The method for obtaining an N-acylated peptide or protein comprises: a) reacting in an aqueous medium a peptide or protein having at least two reactive nucleophilic functional groups with an acylating agent of the formula I; and b) isolating the N-acylated peptide or protein. [n is 0; m is 1-2; w is 4-20; and R1 is a leaving group which facilitates, when reacting the acylating agent with a free amine, forming an amide bond between the carbonyl group attached to R1 and the amine.]【課題】2以上の反応性求核性官能基を有するペプチド又はタンパク質中のアミノ基を選択的にアシル化する方法の提供。【解決手段】a)水溶媒中で、少なくとも2個の反応性求核性官能基を有するペプチド又はタンパク質を式Iのアシル化剤と反応させ、b)N-アシル化ペプチド又はタンパク質を単離することを含むN-アシル化ペプチド又はタンパク質を得る方法。[nは0;mは1~2;wは4~20;R1は前記アシル化剤を遊離のアミンと反応させる際に、R1に結合しているカルボニル基と前記アミンの間のアミド結合の形成を容易にする脱離基]【選択図】なし