A method for enhancing a binding activity of an antibody composition to Fc.gamma. receptor IIIa, which comprises modifying a complex N-glycoside-linked sugar chain which is bound to the Fc region of an antibody molecule; a method for enhancing an antibody-dependent cell-mediated cytotoxic activity of an antibody composition; a process for producing an antibody composition having an enhanced binding activity to Fc.gamma. receptor IIIa; a method for detecting the ratio of a sugar chain in which fucose is not bound to N-acetylglucosamine in the reducing end in the sugar chain among total complex N-glycoside-linked sugar chains bound to the Fc region in an antibody composition; an Fc fusion protein composition produced by using a cell resistant to a lectin which recognizes a sugar chain in which 1-position of fucose is bound to 6-position of N-acetylglucosamine in the reducing end through .alpha.-bond in a complex N-glycoside-linked sugar chain; and a process for producing the same.