The present invention provides gamma-glutamyl-4-azoanilides, processes for their preparation and their use as substrates for gamma-glutamyl transferase determination, of the formula ##STR1## wherein X is alkyl, --(CH.sub.2).sub.m --COOH or --O--(CH.sub.2).sub.n --COOH wherein m is 0-4 and n is 1-4; R is optionally substituted p-nitrophenyl; an optionally substituted thiophene residue; an optionally substituted thiazole residue; an optionally substituted benzothiazole residue; an optionally substituted benzoisothiazole residue; an N-alkylthiazole residue; or an optionally substituted 1,3,5-thiodiazole residue; as well as the alkali metal, alkaline earth metal and ammonium salts thereof.