By estimating serum iron and non-hemin iron of viscera in dogs with experimental hookworm disease, and comparing them with those in normal and blood-depleted anemic dogs used as the control; the following results were obtained: 1. In the highly infected group (macroscopically those showing bloody feces) both serum iron content and non-hemin iron in viscera proved to have decreased, demonstrating a similar tendency as in blood-depleted anemia. 2. In slightly infected group (with intestinal bleeding to the degree of occult bleeding) serum iron content decreases but non-hemin iron content in viscera tends to show the normal or over the normal value. 3. From these data iron deficiency anemia due to disturbance in iron mobilization and bleeding may be pointed out as important factors for hookworm disease.