Abstract: A new method of rapidly analyzing plural substances in the presence of biological catalyzers is disclosed. The method is practiced by way of the steps of injecting both pH buffer solution and specimen into a reaction cell, successively adding a plurality of enzymes to induce uptake reaction of dissolved oxygen, causing the plurality of substances to be subjected to selective oxidation in the stepwise manner, obtaining a stepdown curve of dissolved oxygen by automatically recording the oxidative process by means of a dissolved oxygen sensor, qualitatively determining each of the substances with reference to the kind of added enzymes and the order of their addition and quantitatively determining the same with reference to the extent of decrease in dissolved oxygen. Typically, oxidation of the substances is carried out by way of two or three or further more steps.