It is difficult to make an accurate cytochemical demonstration of the cytochrome oxidase system in the mitochondria of blood cells by the conventional Nadi reaction. By applying the Oda's method for the cytochemical, detection of cytochrome c-cytochrome oxidase system and cytochrome c oxdase the author succeeded in demonstrating cytochemically the cytochrome c oxidase or the cytochrome c-cytochrome oxidase system in the mitochondria of blood cells by using p-phenylenediamine as substrate and neotetrazolium salt as hydrogen acceptor with or without the addition of cytochrome c. This reaction appears in the cytoplasm of all blood cells with an exception of mature erythrocytes in the form of distinct violet-colored round or rod-like granules coinciding with mitochondria. The number, distribution and the grades of the enzyme activity of mitochondria can be seen. whose pictures are specific to each call species. Maturation and differentiation of cells generally proceed with the decrease both in number and enzyme activity.