A highly sensitive assay is described and is characterised by the following reaction stages: 1) a main reaction system in which the coenzyme is transformed according to the amount of the substance to be determined by the action of the oxidoreductase for the said substance in the presence of coenzyme NAD<+> or NADH, or NADP<+> or NADPH; 2) a decomposing reaction system in which the unreacted coenzyme is decomposed and neutralised at the end of the main reaction system; 3) a cycling reaction system in which the reaction is enhanced by means of coenzyme cycling, and which consists of a combination of the oxidoreduction reaction which transforms the excess substrate and coenzyme according to the amount of the substance to be determined, and of a converting reaction of tetrazolium salt to formazan in the presence of diaphorase or of an electron transfer substance and 4) a colorimetric measurement of the formazan formed in this way.
