Healthy lice became infected with Rickettsia prowazeki after feeding on monkeys inoculated with a strain of Mexican typhus. The same result was obtained in 100 per cent of lice by rectal inoculation of an emulsion of tunica vaginalis of guinea pigs reacting to the same strain. In the tunica vaginalis of guinea pigs and rats inoculated intraperitoneally with an emulsion of lice containing Rickettsia prowazeki the intracellular organism constantly associated with the passage strain appeared regularly. Rickettsia prowazeki found in lice and the organism constantly present in the tunica of guinea pigs and rats reacting to our strain of tabardillo are indistinguishable morphologically and tinctorially and their mode of intracellular multiplication is alike in every respect. It is concluded that they are identical. This organism is constantly associated with the causative agent of Mexican typhus, both in mammals and in lice, and all of our attempts to separate them have failed.

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