Selection within a random-mated stock of rats has been effective in isolating lines which continually produced an appreciable increase in the number of individuals resistant to a specific dose of a strain of S. enteritidis over the number that were found in the non-selected parental population.

Considerable variation was noted among the survivors in their ability to transmit resistance to their offspring.

The hybrid progeny from matings between survivors and members of a susceptible inbred race showed a greater resemblance in mortality to that of their resistant parents than to that of the susceptible parental strain. Offspring from matings of hybrid survivors to members of the susceptible strain, and inter se, gave further evidence that hereditary factors were in large part responsible for the resistance displayed in the selected stocks.

The genetic factors operative in the experiments were seemingly multiple, presumably cumulative in effect, and some were at least partially dominant over those making for susceptibility.

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