This invention relates to a method for determining an enzyme-, an enzyme inhibitor-, an enzyme activator-, or zymogen-activity, which comprises allowing the enzyme to react with a substrate of the formula ##STR1## (wherein R.sub.1 represents an amino acid of an oligopeptide combined in the form of ester through the C-terminal carboxyl group of said amino acid or oligopeptide and R.sub.2 represents a hydrogen or bromine atom) to hydrolyze the substrate, then allowing the hydrolysis product of the formula ##STR2## to react with a FR-ITR salt [Fast Red-ITR salt (N,N'-diethyl-4-methoxymetanilamide diazonium salt)] to form a pigment, and determining the pigment.