The purpose of this research is the isolation of an eventual species-specific fraction from the "soluble antigen" of Rickettsiae. The "soluble antigen" of R. prowazeki (Breinl strain), R. typhi (Wilmington strain) and R. canada were purified at 25% saturation with ammonium sulphate (PSA). Corresponding antisera were produced in rabbits. The serological methods used were the complement fixation, the micro-agglutination, the precipitation method in capillary tubes and the immuno-diffusion method carried out with complete and previously cross-absorbed antisera. Furthermore, the PSA were subjected to immuno-electrophoretic and disc electrophoretic fractionation. The PSA of R. prowazeki was found to contain at least 4 group-specific fractions. A species-specific component could be demonstrated with certainty only with the precipitation method in capillary tubes carried out with previously cross-absorbed antisera. The PSA of R. typhi contains 5 fractions of which 4 are group-specific and one is species-specific. This result was confirmed by all methods. The PSA of R. canada: the maximum of 3 components could be detected with the help of immuno-electrophoretic fractionation. A fourth antigenic determinant was made apparent by the presence of corresponding antibodies in the anti-R. canada PSA only.