The antigen analysis of Rickettsia tsutsugamushi has not been studied as yet because of the difficulty in obtaining rickettsiae in a large amount. However, the author could obtain a large quantity of rickettsiae from the peritoneal exudate produced by intraperitoneal inoculation of R. tsutsugamushi into the guinea-pigs previously injected with chlorpromazine. Mitani and Tanizawa strains of R. tsutsugamushi were chosen as the test agents. After homogenizing the exudate for 1 minute, the tissue fraction was separated from it by centrifugation at 3000 rpm for 30 minutes. By salting out of the supernatant of exudate with ammonium sulfate, the albumin and globulin fractions were obtained, and, in addition to these, the polysaccharide fraction was obtained by removing protein with trichloroacetic acid. Each fraction was dialyzed, corrected to the former volume, and then its pH was adjusted to 7.2. Each of the albumin, globulin, polysacchride and tissue fractions did not show any hemolysis in sheep blood cell suspension. In regard to the total protein content, the albumin-globulin ratio, both of which were measured by paper electrophoresis, and LD(50) in mice, no significant difference was observed between the peritoneal exudate containing visible rickettsiae and that not containing them; this result means that the observation of rickettsiae by Gimsa's staining cannot give a exact and definite determination of the infection of the disease.