By the percutaneous or the oral administration of the eggs of dog-ascaris, the dog, the host specific to and guinea pigs and rabbits, the hosts non-specific to the ankylostoma caninum were infected, or reinfected or reinfected after a prior sensitization with the emulsion of ankylostoma caninum; and roentgenograms and histological findings of the lungs of respective cases were compared. The results are described below. 1) In the cases of non-specific hosts, from 5-6 days after the percutaneous or oral infections, they presented abnormal shadows in the roentgenograms of the lungs, and in the case of reinfection on the fourth or fifth day they revealed bronchial pneumonia-like or spotty or nodular shadows: and all these proved to occur much more quickly in a greater degree than the shadow shown in the cases of the primary infection. In the cases reinfected after the sensitization with the emulsion of ankylostoma the shadow of transitory infiltration in the lungs was recognized. In the specific hosts no changes so marked as those observed in non-specific hosts for the primary infection, but in one of the oral reinfection cases among the specific hosts the shadow of the transitory infiltration in the lungs was recognized. 2) Macrospcapically, in the non-specific hosts receiving the oral or the percutaneous primary infections hemorrhages and hyperemia were main changes and in some grayish white nodules could be observed in the lungs. In the cases of reinfection more marked changes and more numerous nodules were observable than in the former, and in the reinfection cases after the prior sensitization with emulsion, grayish white spotty processes were recognized in the lungs. In the specific hosts pathological changes were slight and grayish white spots could be seen only when reinfected. Histologically, in all cases the changes were more striking when reinfected, namely, swelling of the alveolar septum, the nodular cell infiltration and giant cells were observed around blood vessels and bronchi; and in the specific hosts granuloma formation was recognized. 3) From these findings it seems that the hookworm allergy plays one of the pathogenetic roles for the transitory infiltration in the lungs.