A new staining procedure for glucose oxidase at both the light and the electron microscopic level has been developed. Glucose oxidase activity was revealed by a procedure in which the usually employed chromogenic hydrogen donors were replaced by diaminobenzidine. The obtained color is insoluble and osmiophilic and reveals the presence of glucose oxidase activity at either the light or the electron microscopic level. The procedure is equally effective for the detection of galactose oxidase. It appears that the method can be applied to all oxidoreductases which catalyze the production of hydrogen peroxide.