The specificity of Type II pneumococcus determined by its capsular polysaccharide (S II) may be separated into three partial specificities, each characteristic of one of the three component sugars of S II, namely, glucuronic acid, glucose, and rhamnose.

By far the largest portion of antibodies in Type II antipneumococcus horse sera which cross-react with carbohydrates containing one or more of these sugars are reactive with those characterized by multiple groupings of glucuronic acid. This confirms, extends, and explains earlier observations.

In confirmation of predictions based upon existing information tamarind seed polysaccharide (jellose), in which much of the glucose exists as 1,4,6-branch points, was found to react in Type II antisera.

Several instances are given of cross-reactions in these antisera apparently due to the L-rhamnose component of the reacting polysaccharides.

The antisera contain far more antibody capable of precipitating with substances with multiple units of glucuronic acid than with those so far tested containing multiple 1,4,6-branch points of glucose or multiple groupings of rhamnose.

The long known increase of titer of gum arabic on partial hydrolysis is now fully explained and discussed, and a chemical picture is given of the change in "avidity."

The sum of the partial specificities measured does not equal the whole.

Quantitative data illustrating these points are given.

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