The administration of penicillin in relatively large but non-toxic doses to dba mice after injection with murine typhus rickettsiae resulted in a marked reduction in mortality, particularly when the initial dosage of rickettsiae was relatively small, approaching the minimal lethal dose. No evidence of secondary bacterial infection was obtained by bacteriological and histological studies, and it seems justifiable to conclude that the greatly increased survival rate in the treated mice was caused by the action of penicillin on typhus rickettsiae. The theoretical and practical implications of the results are discussed.

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