PURPOSE: Benzoic acid or a hydroxyl-containing benzoic acid derivative is added to a solution for measuring the activity of cholinesterase to obtain high accurate results of cholinesterase activity.

CONSTITUTION: p-Hydroxybenzoic acid hydrogenase and benzoic acid or a hydroxyl-containing benzoic acid derivative such as salicylic acid are added to p- hydroxybenzoic acid resulting from enzymatic reaction of p-hydroxybenzoylcholin as a substrate with cholinesterase to form 3,4-dihydroxybenzoic acid. At this time, the reduction in absorbancy on the oxidation of a coenzyme, nicotinamide-adenine dinucleotidephosphate, from its reduction type (NADPH) into its oxidation type (NADP), is measured at a wavelength of 340nm. Or the consumption rate of dissolved oxygen is measured by means of oxygen electrodes, when p-hydroxybenzoic acid consumes oxygen under action of hydroxylase.