Novel physical means have been used to disintegrate bacterial cells. With the aid of these a new and very labile antigen has been prepared from each of several types of Streptococcus pyogenes (group A, Lancefield). This antigen is the chief component of the surfaces of virulent streptococci in this group. Antibodies against this labile antigen specifically promote agglutination and phagocytosis of, and protect mice against streptococci of corresponding type. Type-specificity may be determined by test-tube agglutination, phagocytosis or mouse-protection with results which are in agreement with type-specificity as determined by Griffith with the method of slide-agglutination. Absorption of sera with labile antigen removes the antibody which determines slide- and tube-agglutination, phagocytosis and mouse-protection. Labile antigen is therefore believed to be the agglutinogen which is the basis of Griffith's classification of types. Labile antigen when detached from the bacterial cell and in solution may be precipitated by immune sera against any type within group A, and by sera prepared by immunization with purified labile antigen itself. Labile antigen may be split by acid hydrolysis into a type-specific fraction and a group-specific fraction which are identical in serologic reactions, respectively, with the type-specific hapten M and the group-specific hapten C. It would appear, therefore, that labile antigen is so oriented, when in the bacterial surface, that the type-specific portion of the molecule is reactive and the group-specific portion is partially or completely unreactive. When in solution the reactions of labile antigen are group-specific. A stable hemolysin-leucocidin has also been isolated from hemolytic streptococci of group A and of other groups. Serologically this substance is a hapten. A crystalline derivative has been prepared which is hemolytic, leucocidic and lethal in small doses for animals. The properties of these substances will be described in detail in a later paper (Czarnetzky, Morgan and Mudd, 1938).