The effectiveness of the Mitomycin C on the reticuloendotherial system is studied in using normal rats and rats with Yoshida-Sarcoma. The serum iron colloid by labelled (59)Fe (S. (59)Fe. C.) administered intravenously is gradually decreased in the circulating blood, being almost disappeared 12 or 24 hours after administration, followed to be gradually increased latsr on again. It has been assumed that the diminition of the (59)Fe from the circulating blood results from the colloid phagocytizing function of the reticuloendothelial system and the succeeding increase of the (59)Fe does from the Hb incorporation. But there is no significant difference in the colloid phagocytizing function of the intravenously injected S. (59)Fe. C., but in the Hb incorporation between normal and treated rats with the Mitomycin C or with the Yoshida-Sarcoma; the Hb incorporation is delayed or inhibitted in the treated rats, while rather improved by the administration of the Mitomycin C to the rats with the tumor. Of the distribution of the administered (59)Fe, which is increased in the spleen, and decreased in the bone in the normal rats with Mitomycin C and decreased in the liver in less degree and in the bone in some degree in the rats with the Yoshida-Sarcoma, compared with the normal rats. Whereas the (59)Fe is decreased in the liver and spleen, and increased in the bone and neoplastic tissue by the administration of the Mitomycin C to the rats with the Yoshida-Sarcoma, compared with the rats with Yoshida-Sarcoma alone. The ratio of Ferritin/Hemosiderin is decreased in the normal rats given Mitomycin C or inplanted Yoshida-Sarcoma, compared with the normal rats, while this ratio rather approached to the normal range in the rats with the Yoshida-Sarcoma given Mitomycin C, compared with the Yoshida-Sarcoma alone. The development of the neoplastic tissue of the rats with Yoshida-Sarcoma has been only temporarily inhibitted with the administration of the Mitomycin C 200 mcg/kg daily for 4 days. However, gradual but steady inhibition is noted with the dosis of 500 mcg/kg per day for 4 days.