Under the conditions specified, there may be selected promptly from a hybrid stock of mice, of which 40 to 50 per cent die following a standard dose of B. enteritidis or St. Louis encephalitis virus, lines in which as high as 95 per cent and as low as 15 per cent succumb. Three lines,-one bacteria-susceptible-virus-susceptible, one bacteria-susceptible-virus-resistant, and one bacteria-resistant-virus-susceptible,-are regarded as remaining relatively stable after approximately twelve generations of selection and brother to sister or line inbreeding. Crossing susceptible with resistant lines and testing F(1), F(2), F(3), and backcross progeny resulted in mortality percentages in the neighborhood of those expected on the basis that resistance to B. enteritidis and to encephalitis virus is each inherited independently on a single factor basis with resistance dominant over susceptibility. A bacteria-resistant-virus-resistant line is being developed from a cross between bacteria-susceptible-virus-resistant and bacteria-resistant-virus-susceptible lines. All selected lines proved uniformly susceptible to a strain of mouse passage rabies virus.