Nitro blue tetrazolium chloride is probably the most commonly used of the tetrazolium salts in dehydrogenase enzyme histochemistry due to the fine precipitate of the formazan. As the enzyme removes hydrogen from the substrate it is transferred to a hydrogen acceptor, usually a co-enzyme. From the co-enzyme it is transferred to the tetrazole, which is consequently reduced to its insoluble and intensely coloured formazan. The formazan of NBT is deep blue and clearly visible.