By studying the effect of hookworm disease serum injection on the iron metabolism in rabbits given such injections and. comparing the same in experimental anemia, the following conclusions have been drawn. 1. In the rabbits given injection of hookworm disease serum, serum iron shows a decrease, while non-hemin iron content in the viscera tends to show an increase in each fraction (4 non-hemin iron fractions of Yoneyama and Konno's method). 2. The results differ from those in phenylhydrazine anemia, collargol anemia and roentgen anemia; and consequently hemolysis, of course, and also a transient disturbance in bone marrow can not be considered as the causative factors of hookworm anemia. 3. From these findings there seems to exist a factor in serum, which inhibits iron mobilization, thus bringing about iron deficiency in the erythropoietic system in bone marrow; and this seems to constitute an important causative factor for the anemia in this disease.