1. Six strains of paratyphoid-enteritidis bacilli, isolated from man and animals, differed markedly in virulence for mice.

2. The inherent virulence of each strain for mice was not affected by repeated intraperitoneal and per os mouse passage.

3. Individual differences in the susceptibility of the mice to each strain and to graded doses of mercury bichloride were demonstrable. Such variations appear to be of a non-specific nature.

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