1. The cross reaction of the specific polysaccharide of Type VIII pneumococcus with Type III antipneumococcus horse serum has been studied quantitatively and found similar to the S III-anti-S VIII reaction.
2. Contrasted with the general similarity of the two-segment reaction curves were distinct qualitative and quantitative differences in the course and character of the reciprocal reactions with respect to each segment.
3. These differences could be interpreted in terms of the known chemical differences between the specific polysaccharides of the two types. A minimum molecular weight of 62,000 was calculated for S III and 140,000 for S VIII.
4. It was also found possible to fractionate the Type VIII antibody into portions characteristic of each segment of the cross reaction curve. At least three different kinds of Type III and Type VIII anticarbohydrates were identified.