Reactivities of various amines in the modifications of acetic acid and Asp-101 of hen egg-white lysozyme in the carbodiimide (1-ethyl-3 [3- (dimethylamino) propyl]carbodiimide hydrochloride (EDC)) reaction were investigated at pH 5.0 and room temperature. The reactivity of an amine towards EDC-activated Asp-101 was at least 50 times higher than that towards EDC-activated acetic acid. Such a high efficiency of modification of Asp-101 could not be explained only by the EDC-binding mechanism in which the EDC molecule binds to the active site cleft of lysozyme close to Asp-101 to activate Asp-101 selectively [R. Kuroki, H. Yamada, and T. Imoto, J. Biochem. (Tokyo), 99, 1493 (1986)]. Therefore, in addition to the above mechanism, an amine-binding mechanism in which an amine molecule binds to lysozyme close to the EDC-activated Asp-101 residue so as to increase the effective concentration of the amine by more than 50 times is proposed.