The kinetic studies of effect of toxoflavine on the action of xanthine oxidase (ECl, 2,3,2) which was extracted from Pseudomona cocovenenans showed that toxoflavin was nonessential activator for the enzyme, and the activation in condition of hypoxanthine as substrate was more marked than xanthine. The activation obeyed the partial activation type. These results provide us a clue for searching toxic mechanism of toxoflavin for human.