Bartonella quintana is a gram-negative microorganism that causes trench fever and chronic bacteremia. B. quintana lipopolysaccharide (LPS) was unable to induce the production of proinflammatory cytokines in human monocytes. Interestingly, B. quintana LPS is a potent antagonist of Toll-like receptor 4 (TLR4), as it inhibited both mRNA transcription and the release of tumor necrosis factor alpha, interleukin 1beta (IL-1beta), and IL-6 by Escherichia coli LPS in human monocytes, at ratios ranging from 1,000:1 to 10:1 (B. quintana LPS to E. coli LPS). Likewise, B. quintana LPS blocked the interaction of E. coli LPS with TLR4 in transfected cell lines. The extent of the inhibitory effect of B. quintana LPS was demonstrated in microarray studies, which showed downregulation of practically all genes induced by LPS in monocytes. Because of the role of TLR4 in inflammation, B. quintana LPS may prove useful as a potent anti-TLR4 agent with therapeutic potential in both infections and autoimmune inflammation.