Intact bacterial membranes have been isolated from protoplasts prepared from Group A streptococci by a cell wall-dissolving enzyme. A membrane fraction with identical chemical and serological properties has been obtained by differential centrifugation of mechanically disrupted streptococci. The membrane is chemically distinct from the cell wall and is composed of 72 per cent protein, 26 per cent lipid, and 2 per cent carbohydrate. Capillary precipitin tests and analysis by microdiffusion have demonstrated that the membrane contains antigens distinct from those of the cell wall and from those of the cytoplasm which it envelops. Evidence is presented which demonstrates that this antigenic material is common to the membranes of Group A streptococci, and that it can be distinguished by immunodiffusion from related antigenic substances present in membranes of several other serological groups of hemolytic streptococci. This antigenic material does not cross-react with the membrane antigens of other Gram-positive cocci.

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