The blood of 63 human subjects selected because of the absence of recent infections, was studied for its content of specific antibodies against virulent strains of Types I, II, and III pneumococci before and after intracutaneous injections of minute amounts of pneumococcus products. The simultaneous injection of the specific polysaccharides of all three types of pneumococci and of proteins and autolysates derived from Types I and II pneumococci was followed by the appearance or increase of pneumococcidal power in the whole defibrinated blood and, in most instances, by the appearance of mouse-protective antibodies and agglutinins for one or more types. A single intracutaneous injection of 0.01 mg. of the protein-free type-specific polysaccharide of either Type I, Type II, or Type III pneumococci or 4 similar daily injections was followed, in most of 29 subjects, by the appearance of antibodies against the homologous, but not against the heterologous type pneumococci. Some subjects showed a simultaneous lowering of a preexisting pneumococcidal power for heterologous or homologous types. A single intracutaneous injection of O.1 mg. of pneumococcus protein in 13 individuals was not followed by the appearance of specific antibodies to any appreciable degree. Single intracutaneous injections of small amounts of autolysates derived from virulent strains of Type I, II, or III pneumococci were followed in 11 subjects by a more or less general rise in the pneumococcidal power with the appearance of homologous type agglutinins and protective antibodies in about one-third of the subjects.