Bartonella quintana has been reported as the cause of trench fever, persistent endocarditis, bacteriaemia and has been isolated with an increasing incidence in clinical specimens from AIDS patients. One of the main pathogenic factors of gram-negative bacteria, including B. quintana, is the lipopolysaccharide (LPS). However, very little information is available on the features of Bartonella LPS. The aim of the present study was to extract, purify and characterise B. quintana LPS. The effect of the LPS under scrutiny was also evaluated on TNFa release by means of the "in vitro" human whole blood model of sepsis. The Oklahoma strain of B. quintana was grown on sheep blood agar, at 37 C, in a moist atmosphere containing 5% carbon dioxide. Cells were harvested and washed in sterile and apyrogenic saline solution and LPS extracted following the procedure of Westphal e Jann (1965), modified by Minnick (1994). The LPS of B. quintana showed the migration pattern of a deep rough chemotype, and the chromogenic limulus amoebocyte lysate test (LAL test) revealed strong reactivity at low concentrations (6.2 pg/ml). Samples of human whole blood stimulated by 1000 ng/ml of B. quintana LPS released 1707 378 pg/ml of TNFa.