The immunogenicity of a number of avirulent rough Salmonella dublin mutants was compared in mice and guinea-pigs. Live vaccine prepared from Strain HB 1/17 at doses of 5 x 10⁷ per mouse usually gave an immunity of between 70 and 80% but in certain experiments the results were more variable and always poorer. This strain gave a cross protection of 28,5% to S. typhimurium in mice. In guinea-pigs it evoked an average protection of approximately 46% to homologous challenge and approximately 26% to challenge with S. typhimurium. Strain 5765 protected up to 80% of mice against S. Dublin infection and was generally superior to Strain HB 1/17 in this respect. It was, however, less effective in protecting mice against S. typhimurium (20%). In guinea-pigs it was also less effective than Strain HB 1/ 17, giving 34% protection against homologous and 20% against heterologous challenge. Other strains also produced immunity in mice but they were not studied in detail. Formalin-inactivated alum-precipitated vaccine prepared from avirulent smooth strain and containing 0, 5% packed cells proved to be extremely effective in protecting mice against S. dublin infection. It produced an average immunity of 75% and was often 100% effective. It also protected 60% of mice against challenge with S. typhimurium. In guinea-pigs it was, however, totally ineffective against challenge with both S. dublin and S. typhimurium.