Using dogs such as ones spontaneously infected with hookworms, ones dewormed of these hookworms, those experimentally infected with hookworms showing marked intestinal hemorrhage as well as normal dogs (the control), and orally administering them radioactive iron, Fe(59), the author determined the radioactive iron in erythrocytes and plasm periodically. Then 48 hours later perfusion is given and fractionating the visceral iron by Yoneyama-Konno's method, the iron contents and Fe(59) are estimated. As the results, it has been found that the iron-absorption rate is lowest in the dogs spontaneously infected with hookworms, followed in an ascending order of normal dogs and dewormed dogs, and highest in the dogs with experimental hookworm disease. As for the distribution of Fe(59) absorbed, in the spontaneously infected dogs it is distributed mostly in the liver, spleen and bone marrow, demonstrating an extremely low portion of it taken into erythrocytes. In the dewormed dogs, on the contrary, the major portion of the absorbed Fe(59) is taken up by erythrocytes and it is sparsely distributed in the liver, spleen and bone marrow, and this tendency is still more marked in the dogs with experimental hookworm disease, Furthermore, the iron contents of various organs in the dogs spontaneously infected with hookworms, though slightly less than in the normal, are greater than those found in the dewormed dogs, proving a marked decrease in the dogs with experimental hookworm disease. From these data it is assumed that there is a definite impediment to the absorption and mobilization of iron in hookworm disease and that such an impediment together with hemorrhage induce anemia.