Groups of mice were immunized with optimal doses of the following vaccines of Salmonella typhimurium W118-2: acetone-killed cells, lipopolysaccharide, ribosomes, and live cells. At 3 weeks, 1 month, 2 months, 4 months, or 6 months postimmunization, sera were collected from control and vaccinated animals, and the anti-lipopolysaccharide and whole-cell agglutination titers of the sera were determined. Other groups of similarly vaccinated mice were tested for resistance to infection by challenging with live W118-2 and scoring the number of survivors 30 days postinfection. It was found that only ribosomes and live cells afforded significant protection 6 months after immunization. Thus, in duration of protection ribosomes were superior to the other nonviable vaccines tested. At all time intervals tested, purified lipopolysaccharide was the least effective vaccine. Protection afforded by the acetone-killed cell and ribosomal vaccines correlated better with the whole-cell agglutination titers than with the anti-lipopolysaccharide titers. However, the longer duration of protection afforded by the ribosomal vaccine, as compared with the acetone-killed vaccine, could not be accounted for by differences in whole-cell agglutination titers. These studies show that ribosomal vaccines are equal in all parameters to acetone-killed cells and have the advantage of providing longer-lasting immunity.