The isolation and characterization of a type-specific M protein from pneumococcus are described. This protein is similar chemically in all respects studied to the M proteins of group A streptococci. No immunological crossreactions have been observed, however, between M proteins of the two species.

Strains of capsular type I pneumococcus have been encountered which contain different M proteins. The same is true for capsular type II pneumococcus. It is apparent, therefore, that the capsular polysaccharides and M proteins can vary independently of each other.

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