1. Under conditions in which mouse typhoid is allowed to spread naturally among herds of mice comprised of different proportions of individuals of innately high or low susceptibility: (a) 85 to 95 per cent of the innately susceptible succumb to mouse typhoid in contrast to less than 5 per cent of the innately resistant, regardless of whether either constitutes 25, 50, or 75 per cent of the population respectively. (b) The surviving population is therefore comprised largely of individuals known at the outset to be innately resistant. These resistants are, nevertheless, apt to have become infected and to harbor mouse typhoid bacilli in their spleens and feces.
2. Under conditions in which recruits are added to surviving populations comprised chiefly of innately resistants among which mortalities have practically ceased: (a) Mouse typhoid infection spreads to both innately resistant and susceptible recruits. (b) Mortality from mouse typhoid is limited almost exclusively to the innately susceptible recruits and is "sporadic" or "epidemic" in character according to the numbers and proportion of susceptibles added. (c) Innately resistant recruits remain well unless subjected to some non-specific hazard, such as heat or overcrowding, in which case both they and the susceptibles succumb in proportions similar to their relative numbers in the population.
3. It was plain that survivors are almost exclusively the individuals known at the outset to have been innately resistant.
4. There was no tendency for known susceptibles to become immunized through herd exposure at epidemic times, at postepidemic times in which the dosage of mouse typhoid bacilli was relatively small, nor at repeated short intervals. Finally, susceptibles given repeated, known, sublethal doses of mouse typhoid bacilli or St. Louis encephalitis virus by a natural route failed to develop immunity against a subsequent test dose.