A new, direct method for determining superoxide dismutase activity is presented in this study. This method is based on measurement of one of the products of the superoxide dismutase reaction, hydrogen peroxide. Hydrogen peroxide is quantitated using a coupled reaction where horseradish peroxidase catalyzes the formation of a fluorescent product, 6,6'-diOH-(1,1'-biphenyl)-3,3'-diacetic acid, from 4-OH-phenylacetic acid and hydrogen peroxide. Substrate for superoxide dismutase is provided by reduction of oxygen during the autoxidation of riboflavin in the presence of UV light. A linear correlation between the amount of superoxide dismutase (200 ng-6 micrograms) and of hydrogen peroxide was found with this method.