BY coupling enzymes to the reduction of a tetrazolium salt, dehydrogenases can be located on electrophoresis strips, and several have been shown to consist of more than one protein component1,2. It is possible to detect other enzymes if their action leads to the formation of a compound which can be oxidized by a suitable enzyme in the incubation mixture. In this way triosephosphate isomerase (TPI) has been located by coupling it with phosphoglyceraldehyde dehydrogenase.