Particulate (P) and soluble (S) antigens of Rickettsia tsutsugamushi for complement fixation were prepared from HeLa cell cultures infected with the Kato, Karp and Gilliam strains. The P antigens of the three strains were found to be type-specific, and the S antigens to be group-specific. The antigens were separated by sucrose density gradient centrifugation, the density (g/ml) of the former being 1.13 to 1.15 and that of the latter 1.06 to 1.08. In animals infected with R. tsutsugamushi, antibody against P antigen was produced earlier and persisted longer in higher titer than that against S antigen. The antigenic structure of R. tsutsugamushi was discussed.