The multiplication of Rickettsia mooseri in fertile eggs is speeded up and quantitatively increased by single dose x-radiation given either 24 hours before or 48 hours after inoculation. This effect is noted at all dosage levels studied, ranging from 100 to 1500 r. The rickettsiostatic effects of high incubation temperature (40°C.) and of streptomycin are neutralized by radiation, but the rickettsiostatic actions of PABA, penicillin, and aureomycin are not altered. Possible mechanisms of action and implications of the observed effects are discussed.

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