Mice were very successfully immunized against intraperitoneal infection with virulent Salmonella typhimurium by fluid and lyophilized formalin inactivated vaccines prepared from a smooth strain of the microorganism. A single subcutaneous injection of 0,2 ml containing 0,1% packed cells was sufficient to confer a solid immunity when tested 2 weeks after immunization. Fluid vaccine was fully potent after storage at 50⁰ C for 1 month. A good immunity was also obtained with live vaccines prepared from rough mutants of S. typhimurium. The degree of protection varied with the vaccine strain used. Only those mutants which possessed some degree of residual virulence produced a solid immunity while absolutely avirulent mutants were ineffective unless excessively high doses were employed. Neither inactivated nor live S. typhimurium vaccines conferred protection against S. dublin infection.