The mutual relationship of the anticarbohydrate precipitins and the protective action in antipneumococcus sera to the soluble specific substance was investigated. The assumption is made that there exists in antipneumococcus serum, type-specific protective antibody which is distinct from the anticarbohydrate precipitins and is not neutralized by the soluble specific substance. This assumption is based upon the following observations in experiments which were conducted primarily with Type I antipneumococcus horse serum.
1. Lack of proportionality between the quantity of SSS added and the amount of anticarbohydrate precipitin and protective action neutralized.
2. Protective capacity of specific precipitates (SSS-precipitin complex) is accounted for on the basis of a liberation of non-specifically adsorbed protective antibody.
3. SSS only partially neutralizes the protective action of antipneumococcus serum in vivo.
4. Type-specific protective antibody remains in antipneumococcus serum after complete precipitation of the anticarbohydrate precipitins.
5. This residual type-specific protective antibody is not neutralized by additional SSS nor by absorption with heterologous pneumococci; it is definitely absorbed by the homologous pneumococci.