1. Mice infected with Anchylostoma caninum larvae labeled with Fe(59) showed anincrease of both radioactivity and the number of the larvae in the lungs 12 hours after the infection. 2. Mice orally infected with Anchylostoma caninum larvae were treated beginning 24 hours after the infection with chloroquine diphosphate, Stibnal and Mapharsen. Chloroquine diphosphate showed a marked anchylostomocidal effect against the larvae migrated into the mouse lungs. Stibnal and Mapharsen were also fairly effective. 3. On the basis of the in vitro anchylostomocidal effect, clinical trial and animal experiment using mice, it is believed that these drugs severely damage hookworm larvae, especially those in the lungs, and the etiology of the lung symptoms of Wakana disease can be largely accounted for by the mechanical irritation of the larvae, although it is probable that they have something to do with an allergic phenomenon.