A fatal infection of irradiated white mice with the Breinl strain of European typhus has been established and passed serially for 22 passages by the intra-abdominal route. Rickettsiae were abundant and easily demonstrable in the moribund or dead mice.

The mortality of irradiated mice infected with passage material (peritoneal washings or blood) was nearly 100 per cent as contrasted to no mortality in the control mice given the same dose of x-ray (450 R) and the same volume of fluid intra-abdominally. (The observation period of control mice was arbitrarily limited to 14 days.)

After eighteen passages in irradiated mice no increase in virulence for non-irradiated adult mice was detected.

After passage in guinea pigs, the rickettsial infection deriving from the mouse passage material was identical with the Breinl strain as judged by fever, cross immunity tests, and brain lesions in sections.

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