In the present experiments, Mitani, Tanizawa, Ohzeki and Shichito strains of Rickettsia tsutsugamushi were used. Rickettsiae of each strain were purified with kaolin, and their antigenic properties in the complement fixation test were investigated in detail. The results were as follows: 1) In the complement fixation test between the purified rickettsiae of Mitani strain and the antiserum of Mitani or Ohzeki strain, marked complement fixation was observed. 2) In the complement fixation test between the purified rickettsiae of Tanizawa strain and the antiserum of Tanizawa or Shichito strain, complement fixation was observed to a high degree. 3) The purified rickettsiae of Ohzeki strain showed marked complement fixation in the antiserum of Ohzeki or Mitani strain. 4) The purified rickettsiae of Shichito strain showed a high degree of complement fixation in the antiserum of Shichito or Tanizawa strain. Namely, when purified rickettsiae were used as the antigen of the complement fixation test, a significant inhibition of hemolysis was observed to a higher dilution of the antiserum of the homologous strain than that of the heterologous strain.