Ribosomal vaccines prepared from Salmonella typhimurium SR-11 and 6707 an Re mutant bacterium of strain LT2, were effective immunogens in A/J and C3H/HeDub inbred mice. Only SR-11 ribosomes were able to induce significant protection in C3H/HeJ mice. C57BL/6J mice were not protected by either ribosomal preparation. A/J mice were protected against salmonella infection by purified SR-11 endotoxin preparations. Neither the C3H/HeDub, the C3H/HeJ, nor the C57BL/6J mice were protected by the endotoxin preparation. Passive hemagglutination studies showed that C3H/HeJ mice had no antibodies to O antigen but were significantly protected by SR-11 ribosomes. In contrast, C57BL/6J mice, which had the highest titers of O antibodies of the four inbred mouse strains, were not protected by SR-11 ribosomes. Endotoxin cannot totally account for the effectiveness fo ribosomal account for the effectiveness of ribosomal vaccines prepared from S. typhimurium.