1. Immunization of rabbits with Type III pneumococci is effective in producing active immunity against infection with a virulent strain of the homologous organism. 2. Immunization of rabbits with Type I or II pneumococci, and with R forms derived from any of the fixed types, is equally effective in producing active immunity against Type III infection. 3. Immunization of rabbits with nucleoprotein or with desoxycholate solutions of heterologous pneumococci, under the experimental conditions described, appears to be ineffective in producing active immunity against Type III infection.