Bartonella quintana is transmitted by body lice among homeless people. Infection with this organism leads to chronic bacteraemia with few symptoms. We looked for B quintana in erythrocytes in a population of homeless people in Marseille, France. In this report we show the intraerythrocytic presence of B quintana in people with symptomless bacteraemia, by use of a specific monoclonal antibody and laser confocal microscopy. Presence of at least 5 x 10(4) colony-forming units per mL was predictive of intracellular infection. However, such infection was not associated with evidence of haemolysis. Colonisation of erythrocytes could allow efficient transmission of B quintana by body lice.