The effect of x-rays on the growth of rickettsiae in the embryonate egg was investigated.

The intensifying effect of x-radiation of the host on rickettsial growth can be attributed to alterations of the cells of the host that persist at least 7 days.

The greatest enhancement of the growth of rickettsiae occurred when moderately infected cells were irradiated.

These experiments indicate that x-rays may neutralize or reverse changes in the host that are unfavorable to the growth of rickettsiae.

Explanations of the observed phenomena in terms of biochemical and biological alterations of the cells of the host are discussed.

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