Using Ozeki strain, Mitani strain, XXII strain of rat, Tanizawa strain and Shichito strain and their corresponding antisera, the author observed the skin reaction by the intracutaneous inoculation of the alumina gel-treated rickettsia in the rabbits preimmunized by the infection of each strain. By these cross-neutralization test and skin reaction-inhibition test, immunological relationship among these strains was studied. The results are as follows : 1. Ozeki strain, Mitani strain and XXII strain of rat are immunologically allied to one another, namely cross-neutralization is established among them, and the skin reaction is inhibited by one another in the rabbits pre-immunized by the infection of each strain. 2. Cross-neutralization is established between Tanizawa and Shichito strains, and, in the rabbits pre-immunized by the infection of the one strain, the skin reaction is inhibited by the other ; these two strains are also immunologically in near relation to each other.