1. A group of 72 human subjects were studied with respect to the immune reactions of their blood and sera to Types I, II and III pneumococci and to 4 other types (V, VI, VIII and IX) previously included in Group IV.
2. The same general relationships were observed for all of these types as were previously demonstrated for Types I, II and III. Each type was specific in relation to the bactericidal action of normal human blood and the protective action of normal human serum.
3. The frequency with which pneumococcidal action for any pair of types was present for both or absent for both in the same blood samples was slightly greater than that calculated from the frequencies with which each of the types was killed separately.
4. No closer correlation could be demonstrated between the reaction of the blood of these subjects to Types II and V or between Types III and VIII pneumococci, types related in their reaction with artificially prepared immune sera, than was observed between unrelated strains.