Alcohlic groups of serine(Ser) that are located in anti-human IgM(Fcμ) (rabbit Ig fraction) specific site are selectively activated by phenylmethylsulfonyl fluoride (PMSF) and displaced with hydrogen selenide (H2Se). Thus, Ser residues can be chemically converted into selenocysteine (SeCysh). The modified antibody has glutathione peroxidase (GSH-Px) activity which is seventy times more than that of PZ51, the best mimic known in the world.The titer of modified antibody is similar to that of the unmodified antibody.