a method for assay of enzyme activity which comprises treating an amide compound of the formula ## str1 ## with peptidase ; treating thus - liberated amine of the formula ## str2 ## with an oxidase which consume oxygen and forms pigment by oxidative condensation of said amine with a coupler of the formula ## str3 ## and quantitatively measuring the detectable changes . also disclosed is an assay method which comprises treating an amide compound of the formula ## str4 ## or a salt thereof , with peptidase to liberate an aniline derivative of the formula ## str5 ## oxidizing said aniline derivative ; and quantitatively measuring detectable change .