Mice which survive a preliminary dose per os of paratyphoid-enteritidis bacilli are more resistant to a second dose per os of a similar epidemic mouse typhoid strain than are mice which have received no preliminary culture.

The amount of this resistance is related to the pathogenicity of the preliminary strain more than to its antigenic similarity to the second strain.

Mice which survive a preliminary dose per os of paratyphoid-enteritidis bacilli are more resistant to a lethal dose of mercury bichloride per os than are mice which have not received the preliminary bacterial culture.

Mice which have resisted and mice which have recovered from a preliminary dose per os of paratyphoid-enteritidis bacilli are more resistant to a lethal intraperitoneal dose of an epidemic mouse typhoid strain than are mice which have not received the preliminary culture.

Mice in which the preliminary dose of paratyphoid-enteritidis bacilli has induced a chronic infection readily succumb to a second dose of such bacilli.

These findings indicate that the resistance mechanism of the host contains important non-specific factors which vary in degree with the individual mice.

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