PAAG-electrophoresis of the isogenic pair of Rickettsia prowazekii strains E and Evir lysates demonstrate the similarity in polypeptide tracks. The different electrophoretic mobility of the Mr 30 Kd protein from these strains as compared with the mobility of analogous protein from the standard virulent Breinl strain is registered. In immunoblot experiments the specific rabbit antiserums obtained on the 30th day of infection with the Breinl, E or Evir strains demonstrate the presence of the different main antigens 60 Kd or 70 Kd. The difference evidently reflects the specificity of development of two forms of infection by the strains having different virulence. The surface tris-soluble antigens of Rickettsia prowazekii have the similar polypeptide contents and immunochemical properties. The main component of tris-soluble antigens Mr 130 Kd protein is not strain specific having the common thermolabile epitope.