A study was made of representative strains of type XI and of type XVI pneumococci and of a number of strains which exhibited cross-reactions with serum for both these types. By tests for agglutination, passive protection in mice, capsular swelling (8) and absorption of agglutinins, the strains exhibiting cross-agglutinations were shown to be distinct from either type XI or type XVI but immunologically related to both.

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This study was aided, in part, by a grant given in honor of Francis Weld Peabody by the Ella Sachs Plotz Foundation.

The work was carried out with the technical assistance of Mrs. Mildred W. Barnes.

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