1. Immunization of man with 0.03 to 0.06 mg. of each of the capsular polysaccharides of pneumococcus types I, II, V, and VII, given in a single subcutaneous injection, has been shown to be effective in preventing pneumonia caused by these types but not that due to heterologous types. 2. Immunity appears within a period of 2 weeks following injection of the polysaccharides. Its duration was not determined, although 6 months can be set as a minimum. 3. Immunization of alternate subjects in the population reduced greatly the incidence of pneumonia in the non-immunized. 4. The carrier rate for pneumococcus types I, II, V, and VII was lowered significantly in the immunized group as compared with the controls. It is suggested that an over-all reduction in the incidence of carriers was responsible for the lowered rates for pneumococcal pneumonia in the non-immunized group.