In a process for measuring the activity of a dehydrogenase or the amount of substrate reacted in the presence of the dehydrogenase using NAD or NADP as coenzyme and a tetrazolium salt as a color-producing reagent, when at least one member selected from the group consisting of dodecyl sulfate, decyl sulfate, dodecylbenzenesulfonic acid, and salts thereof is used as a reaction stopper, resulting in enhancement of coloring, stability of the produced color and prevention of the produced formazan compound from staining the laboratoryware.