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 tetrazorium 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, deposition and staining of a formazan compound produced can be prevented and improvement in color-producing stability and enhancement of coloring can be attained.