Bartonella quintana, the agent of trench fever and a cause of endocarditis and bacillary angiomatosis in humans, has the highest reported in vitro hemin requirement for any bacterium. We determined that eight membrane-associated proteins from B. quintana bind hemin and that a ∼25-kDa protein (HbpA) was the dominant hemin-binding protein. Like many outer membrane proteins, HbpA partitions to the detergent phase of a Triton X-114 extract of the cell and is heat modifiable, displaying an apparent molecular mass shift from approximately 25 to 30 kDa when solubilized at 100°C. Immunoblots of purified outer and inner membranes and immunoelectron microscopy with whole cells show that HbpA is strictly located in the outer membrane and surface exposed, respectively. The N-terminal sequence of mature HbpA was determined and used to clone the HbpA-encoding gene (hbpA) from a lambda genomic library. The hbpA gene is 816 bp in length, encoding a predicted immature protein of approximately 29.3 kDa and a mature protein of 27.1 kDa. A Fur box homolog with 53% identity to the Escherichia coli Fur consensus is located upstream of hbpA and may be involved in regulating expression. BLAST searches indicate that the closest homologs to HbpA include the Bartonella henselae phage-associated membrane protein, Pap31 (58.4% identity), and the OMP31 porin from Brucella melitensis(31.7% identity). High-stringency Southern blots indicate that all five pathogenic Bartonella spp. possess hbpAhomologs. Recombinant HbpA can bind hemin in vitro; however, it does not confer a hemin-binding phenotype upon E. coli. IntactB. quintana treated with purified anti-HbpA Fab fragments show a significant (P < 0.004) dose-dependent decrease in hemin binding relative to controls, suggesting that HbpA plays an active role in hemin acquisition and therefore pathogenesis. HbpA is the first potential virulence determinant characterized fromB. quintana.