The prior observation that Rickettsia-like microorganisms could be propagated from the blood of a patient with trench fever directly on blood agar received confirmation by further isolations of rickettsiae on cell-free media from the blood of two volunteers infected with a Mexican strain of Rickettsia quintana. The first volunteer was infected by scarification with saline suspensions of body lice, randomly collected in México City, in whose feces R. quintana had been seen. Infective blood from this patient was inoculated subcutaneously into the second volunteer. Both volunteers contracted typical clinical trench fever. The etiology of their infection was confirmed by xenodiagnostic louse-feeding. R. quintana was propagated from the blood of both volunteer patients directly on blood agar. Lice inoculated with the microorganisms so cultivated showed a typical histologic picture of exclusively extracellular rickettsiae in the lumen of their gut.