The cytoplasmic granules of the blood neutrophil leukocyte of the teleost, Ictalurus punctatus, have been shown to exhibit peroxidase activity at the light and electron microscopic levels when exposed to the 3,3′-diaminobenzidine tetrahydrochloride peroxidase procedure. Erythrocytes also show activity. The addition of cyanide to the incubation medium has no apparent effect on peroxidase reactivity in neutrophils, but inhibits pseudoperoxidase reactivity in erythrocytes. The presence of peroxidase-positive granules in the neutrophil serve as a marker for identification of this cell and strongly indicate antibacterial and phagocytic functions for the neutrophil.