1. A colony of inbred mice has been established free from inter-current infections and from the influence of extraneous variables detrimental to a study of their resistance to natural infection. A technique of measuring the resistance of mice to natural B. enteritidis mouse typhoid infection has been developed. 2. The Rockefeller Institute albino strain showed an average 37.4 per cent mortality to B. enteritidis mouse typhoid infection. 3. By breeding from progeny of mice dying early after exposure, lines approximately 85 per cent susceptible were obtained; by breeding from unexposed progeny of mice surviving infection, lines 15 per cent susceptible were obtained. Further selection within the most susceptible and most resistant lines did not alter their reaction; selection within the less susceptible and less resistant lines increased the sought factor. These rates remained approximately the same when the susceptibles received 1/100th and the resistants ten times the standard dose. The selected mice reacted consistently when exposed to spontaneous herd infection. 4. Several strains of mice, brother to sister inbred for special factors, did not differ materially in susceptibility. White-face and selected black-and-tan strains, however, proved 89.0 and 97 per cent susceptible. 5. White-face susceptibles mated with resistant Rockefeller Institute mice gave F(1) progeny, 17.7 per cent susceptible. F(1) mice back-crossed to susceptibles gave progeny 61.2 per cent susceptible and back-crossed with resistants gave progeny 26.3 per cent susceptible. F(2) mice mated inter se gave progeny 34.2 per cent susceptible. The results indicate that resistance factors are dominant and not sex-linked.