The hemAmutation reduces the virulence of Salmonella typhimurium for mice by at least 107-fold, as measured by change in LD50. The hemAmutation does not appear to affect killing of salmonella in mice. The salmonella with the hemAmutation persist in the spleen and liver for 2 to 3 weeks following intravenous injection. The most likely effect of the hemA mutation is to block, or retard, growth of S. typhimurium in an aerobic in vivo environment. Intravenous vaccination of susceptible Itys mice with hemA salmonella was able to elicit about 4 logs of protection against invasive infection with wild-type S. typhimurium 78 days after vaccination, at a time when the vaccine strain was no longer detectable in the spleen and liver. © 1991.