1. Mice with relatively great inherent resistance to certain bacterial infections were heavier but not more fertile than mice with relatively little inherent resistance. Mice with relatively little inherent resistance were with one exception not abnormally low in weight.
2. B. enteritidis given intrastomachally to susceptibles appeared in the blood stream more promptly, in larger numbers, and in a greater percentage of cases, and was present in feces in larger numbers, for a longer period, and in a greater percentage of cases than when given to resistants.
3. Mice relatively resistant to B. enteritidis administered by the natural gastrointestinal route were likewise resistant to the organisms introduced subcutaneously, intraperitoneally, and intravenously. Mice relatively susceptible to the organisms administered by the natural route were susceptible when the organisms were injected directly into tissues and blood stream.
4. Of four lines of mice relatively susceptible to B. enteritidis, three were likewise susceptible to Pasteurella avicida, B. friedlaenderi, and pneumococcus given intranasally. A fourth line was significantly more resistant. Lines of mice relatively resistant to B. enteritidis were likewise resistant to the three respiratory tract pathogens.
5. When Pasteurella avicida, B. friedlaenderi, and pneumococcus were injected intraperitoneally or intravenously, no significant differences in duration of life of the several lines of mice could be demonstrated.
6. Of four lines relatively susceptible to B. enteritidis, two were susceptible to an intranasal instillation of louping ill virus. Lines resistant to B. enteritidis proved relatively susceptible to the virus infection.