1. The reaction between the specific polysaccharide of Type III pneumococcus and homologous antibody in rabbit sera is quantitatively accounted for by expressions similar to those derived from the mass law for the corresponding horse sera. Preliminary data are also given for the Type I reaction.
2. Differences and similarities of the reaction with antibodies produced by the two animals are discussed.
3. Calculations are made of the equivalent composition of the specific precipitate at various reference points in the reaction range.
4. Certain theoretical and practical implications of the findings are pointed out.