In the field of hematology the supravital staining by Janus green has been used extensively for counting of the number of mitochondria of leucocytes. But it has been clarified that all the mitochondria can not be stained by this method. With the purpose to establish a method to stain all mitochondria the author has scrutinized the histochemical method for the respiratory system, especially the succinic dehydrogenase system, localizing solely in mitochondria and devised the color reaction to appear coinciding fairly exactly at the site of mitochondria improving the past method. For the enzyme reactions sodium succinate was used as the substrate and neotetrazolium chloride, nitro-neotetrazolium chloride or nitiro-blue tetrazolium chloride as the hyrogen acceptor. In this paper by the applicatiou of this method the author stuied the correlation between the site and intensity of the reaction in each cell species with reference to the differentiation and maturation. The results are as follows: 1. The cytochemical reaction of this enzyme appears positive in all blood cells with exception of mature erythrocytes. This enzyme reaction decreases its intensity along with the maturation and differentiation of cells. 2. The enzyme reaction appears specifically coinciding with the site of mitochondria in cells including reticulocytes. 3. This enzyme reaction is extremely useful for counting the number of mitochondria in blood cells as well as for ascertaining their enzymatic activity.