Inbred mouse strains show characteristic susceptibility to S. typhimurium infections. The sensitivity of the same strains to endotoxin is not parallel. While C3H/He J and C57B1/6J mice were highly susceptible to infection they showed more resistance to purified endotoxin than A/J, a mouse strain relatively resistant to infection. These findings suggest that the death of mice during S. typhimurium infection is not due only to toxic effects of its lipopolysaccharide.