Bartonella quintana, the aetiologic agent of trench fever, has recently been implicated in culture-negative endocarditis and bacteraemia amongst homeless people. B. quintana is a fastidious slow-growing organism. A tissue culture system of human endothelial cells was developed in which B. quintana grew intracellularly. Observation of the different steps during infection of these cells demonstrated that the bacteria adhered to and penetrated the cells by phagocytosis. During the preadherence stage, most bacteria exhibited surface appendages that resembled those described for Salmonella typhimurium and which may mediate specific interactions between the eucaryotic cell and the bacterium. Soon after the engulfment step, the bacterium appeared in a cell vacuole where it multiplied, giving the typical aspect of morulae which has also been reported with Ehrlichiae or Chlamydiae. In older cultures, the coexistence of bacteria and huge quantities of vesicle-like structures in the same vacuole were noted. These vesicle-like structures were also found with agar-grown bacteria and were identified as membrane blebs. Microscopic observation of heart valves from B. quintana endocarditis patients demonstrated the intracellular location of B. quintana in vivo. This intracellular location of B. quintana should now be considered in further studies on the pathogenesis of the diseases it causes.