1. Type-specific antipneumococcus immunity has been induced in rabbits by immunization with antigen prepared by combining a specific derivative of the capsular polysaccharide of Type III Pneumococcus with globulin from horse serum.
2. Rabbits immunized with this antigen acquire active immunity against infection with virulent Type III pneumococci.
3. The sera of the immune rabbits contain type-specific antibodies which precipitate the Type III capsular polysaccharide, agglutinate Type III pneumococci, and specifically protect mice against Type III infection.
4. The experimental data are discussed with reference to: (1) the concurrence in the immune sera of type-specific antibodies for Pneumococcus and precipitins for horse globulin; (2) the determining influence of the capsular polysaccharide on the specificity of the antigen as a whole; (3) the unity of the type-specific precipitins, agglutinins, and protective antibodies induced by a single compo of the pneu mococcus in chemical union with an unrelated, animal protein.