Serologically active preparations of R. prowazekii membranes were obtained by the lysis of purified R. prowazekii with ether and by differential and gradient centrifugation. Purified R. prowazekii and their membranes were analyzed by the method of electrophoresis in acrylamide gel. The former contained not less than 30 proteins with molecular weights of 10 000-169 000 daltons, while the membrane preparations contained 5 main polypeptides with molecular weights of 12400, 21500, 29600, 34000 and 133600 daltons. Antisera obtained after the immunization of rabbits with the membrane preparation were found to contain antibodies reacting in the complement fixation test and neutralizing rickettsial toxin.