By means of bone marrow tissue culture the author investigated the effects of cytochrome C on leukopoietic function and obtained the following results: 1) When cytochrome C was added directly to the bone marrow tissue culture of normal guinea pigs, normal persons and patients with various diseases the growthrate of outgrowth, the wandering velocity and carbon particle phagocytosis of neutrophils were increased as compared with those of the control added with saline. As the result, it has been seen that by adding an optimal amount of cytochrome C the leukopoietic functions of bone marrow were accelerated. 2) When cytochrome C was added with carcinostatic substances to the bone marrow tissue culture of normal persons, the growth rate of outgrowth, the wandering velocity and the carbon-particle phagocytosis of neutrophils were increased as compared with those of the control added with only carcinostatic substances. The similar results were obtained in the case of bone marrow tissue culture of guinea pigs of which bone marrow had been made hypoplastic by repeated administration of carcinostatic substauces and then injected with the repeated administration of cytochrome C. On the other hand, in its clinical applications, the repeated administration of cytochrome C were effective on some cases of leukopenia induced with carcinostatic substances. As it has been demonstrated that cytochrome C has some relieviug effects on the disturbance of leukopoietic functions of bone marrow successively injected with carcinostatic substances.