Chemical extraction of lyophilized and ground staphylococci of type A has yielded a type-specific, antigenic fraction which has been shown to be protein. In precipitin tests this fraction reacts in high dilution with homologous immune sera prepared by the injection of animals either with the whole organisms or with the protein itself. Reactions with sera prepared against heterologous types of staphylococci occur only in low dilutions. The type specificity of this fraction has been found to be entirely independent of that of the type-specific carbohydrate described by Julianelle and Wieghard.

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