By injecting to normal rabbits the spleen extracts and sera of the rabbits with experimental splenomegaly, the author examined whether or not the toxic factor exists in such extracts and sera, and studied influences of the toxic factor upon the iron and copper metabolisms. The experimental results are described below. 1) A toxic factor was recognized to exist in the spleen and serum of the rabbits recei. ving the injections of methylcellulose, polyvinylalcohol and ovalbumin. 2) By the injection of the substances containing such a toxic factor to the normal rabbits, the value of serum iron became lower while that of copper higher. In studying the direction of action of the toxic factor mainly from the standpoint of the iron metabolism it has been found that despite the normal amount of depot iron, the serum iron decreased markedly. From this fact it is concluded that the anemia is caused by the inhibitory action of the toxic factor on the mobilization of depot iron with resultant interferance with the erythropoiesis in the bone marrow. 3) As mentioned above, the author believes beyond doubt to have confirmed for the first time the existence of a toxic factor in the spleen and serum in the rabbits with experimental hypersplenism and to have clarifed the active mechanism of the toxic factor.