The cytoplasmic granules of the human basophil leukocyte have been shown to exhibit peroxidase activity both at the light and electron microscopic levels when exposed to the 3,3’- diaminobenzidine tetrahydrochloride peroxidase procedure. Reactivity is diminished when exogenous peroxide is omitted from the incubation medium and is completely abolished when catalase is included in the peroxide-free medium