Polyol dehydrogenases are enzymes that convert polyalcohols into sugars, using NAD+ as hydrogen acceptor. Sorbitol, mannitol or other polyalcohols can be used as substrates and NADH is produced. In this assay, proteins are first separated by native polyacrylamide gel electrophoresis and the gel is then incubated with the enzyme substrates. NADH produced by the dehydrogenase reduces tetranitro blue tetrazolium chloride (TNBT), generating a black product that stains the gel. This procedure gave good results with polyol dehydrogenases from the digestive gland of gastropods and, with the proper adaptations, can be applied to other dehydrogenases and other organisms.