Glycogens from various animal and vegetable sources precipitate antipneumococcal horse sera of Types II, VII, IX, XII, XX, and XXII.

Fractionated glycogen and glycogen recovered after reprecipitation, acetylation, and deacetylation precipitate the antisera, but glycogen degraded by saliva does not.

A fraction of the antibody is precipitated in the antisera by glycogen. Possible chemical relationships accounting for these instances of cross-precipitation are discussed in terms of the structures of glycogen and the type-specific polysaccharides of pneumococcus and the quantitative theory of specific precipitation.

Amylopectin also gives cross-reactions of smaller magnitude. Quantitative data on these are withheld until irregularities have been eliminated.

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