Abstract: Leucine aminopeptidase (LAP) activity or .gamma.-glutamyltranspeptidase (.gamma.-GTP) activity in a sample is determined by converting a substrate represented by the formula (I): ##STR1## wherein Z represents (CH.sub.3).sub.2 CHCH.sub.2 CH(NH.sub.2)--or (NH.sub.2)(COOH) CHCH.sub.2 CH.sub.2 --, R.sub.1 represents hydrogen, alkyl or substituted alkyl, R.sub.2 represents alkylene or hydroxyalkylene, R.sub.3 and R.sub.4 are the same or different and represents hydrogen, halogen, nitro, hydroxyl, sulfo, carboxyl, alkyl, and alkoxy, and salts thereof into a compound represented by the formula (II): ##STR2## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 have the same meanings as defined above. The compound (II) is reacted with a chromogen to form a pigment or converted into a diazonium salt which is reacted with a coupling agent to form an azo dye. Absorption of a resultant colored reaction solution is measured.