The oxidation of glucose-6-phosphate, 6-phosphogluconate and ribose-5-phosphate in the presence of yeast autolysate is promoted by addition of vitamin K_3 as well as other derivatives of naphthoquinone. The comparative activities of naphthoquinone and its derivatives are shown. The oxidation of succinate, lactate and alcohol by animal tissue is also promoted by addition of vitamin K_3. In the systems in which diphosphopyridine nucleotide (DPN) or triphosphopyridine nucleotide (TPN) is concerned, vitamin K_3 acts as a hydrogen carrier, passing on hydrogen of reduced DPN or TPN to oxygen in the presence of yellow enzyme.