A Mexican strain of Rickettsia quintana isolated directly on blood agar from a volunteer trench-fever patient and passaged exclusively on this medium induced typical clinical trench fever in two of three volunteers. The etiology of their infection was established by xenodiagnosis as well as by recovery of the rickettsiae from their blood directly on blood agar. The recovered microorganisms produced typical extracellular infection in lice after intrarectal inoculation. The numbers of rickettsiae circulating in the peripheral blood of the two infected volunteers at various intervals after onset of disease were quantified. Both infected volunteers had antibodies in low titer, which reacted with a complement-fixing antigen prepared from a European strain of R. quintana cultivated on blood agar. The experiments established that the microorganisms propagated on cell-free media from the blood of trench-fever patients were R. quintana.