N-[5-[1-Hydroxy-2-[[2-(4-methoxyphenyl)-1-methylethyl-d6](phenylmethyl)amino]ethyl]-2-(phenylmethoxy)phenyl]formamide (E)-2-Butenedioate (Salt) is the labeled analogue of N-[5-[1-Hydroxy-2-[[2-(4-methoxyphenyl)-1-methylethyl-d6](phenylmethyl)amino]ethyl]-2-(phenylmethoxy)phenyl]formamide (E)-2-Butenedioate (Salt), which is used for the preparation of 3-acylamino-4-hydroxy-α-(N-substituted aminomethyl)benzyl alcohol derivatives as β-adrenoreceptor stimulants.