In order to study the influences of hookworm toxin on the iron and the copper metabolisms in vivo, observations were carried on the fluctuations of the iron and the copper contents in rabbits injected each of anemizing substances such as emulsions of hookworms and hookworm larvae and serum of hookworm patient into rabbits; and the following results were obtained. 1) In the cases receving injection of either hookworm emulsion or larvae eumulsion, the iron content in blood decreases markedly, whereas the copper content on the contrary increases remarkably. 2) In the cases given intravenous injection of hookworm serum, the iron content in blood decreases markedly but that of copper only slightly. 3) In the cases given injection of emulsions of hookworm and hookworm larvae, and in the cases administered with serum of hookworm disease, both present different results of influences upon the copper content in blood. This fact seems to indicate that anemizing factors in the latter substance act directly on the copper content as they are, while those in the formerneed to be activated by the liver before exerting any influences upon the copper content in blood.