In the present section, the author studied comparatively the death by the lethal toxin and the death by the infection of the virus itself. The caudal vein of the mouse was chosen as the site of inoculation, and the establishment of infection was judged by observing the pathological changes of the inoculated mouse. The results were as follows: 1) By the inoculation of the virus into the caudal vein of the mouse, beside the lethal toxin which killed the mouse within about 24 hours, a toxicity was observed to which the mouse succumbed a few days after the inoculation. This toxicity was proved in all tested strains, though somewhat varied according to the strains of the virus. Some cases were observed, however, in which the inoculated mice survived. 2) These two sorts of toxicity were well demonstrated in the chick embryo, particularly in the liver. The emulsion of other materials than the liver showed but a little toxicty. It is inferred, therefore, that the liver of chick-embryo plays a very great role in the manifestation of the toxicity. 3) When the multiplication of the virus was very well, the toxicity of these two kinds was very high. When the multiplication of the virus was bad, the toxicity was low and mice survived. 4) In the case of the death by the lethal toxin, the typical pathological findings could not be observed. In the case of the death by infection, however, the typical prthological changes were observed in high degree. Even in the surviving mice, the pathological changes were slightly observed.