Studies on the theoreis of immunity against experimental typhoid have so far been almost confined to the infection with smooth type organisms. I have reported in the preceding report that the variation of mutabile type can be originated from rough phase of Salmonellae, and in this variation, the changes in virulence are identical with those in the variation of mutabile type originated from S phase, that is, the virulence of mutabile variant originated from R phase becomes markedly attenuated when compared with the original R type organisms. Thus following experiments were performed to see the influences of immunization with this atte zuated mutabile variant or killed R type organisms against experimental typhoid caused by rough variant of Salmonellae possessing somatic antigen which is conventionally considered as an unimportant component for their virulence or pathogenicity, and to see, in case there exists any difference between the influences of these two immunizing procedures, to what mechanism such difference is ascribable.
There are numerous bases for the evaluation of immunizing effects against experimental typhoid, but in the present experiments the fate and the distribution of infected bacilli in mice are chiefly adopted as a basis for the evaluation of immunizing effects. And to see the distribution of infected bacilli in the animal tissues, examinations were made on the specimens obtained from infected animals sacrificing them daily, and in order to see the fate of infected bacilli in one animal, examinations were also made on the tail blood and peritoneal fluid of alive infected animals sampling them daily. On the other hand, the experiments concerning the immunity against the primary sepsis caused by inoculation of a massive dosls of R type variant were also carried out.

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