Lipopolysaccharides (LPS), or endotoxins, are major structural and functional components of the outer membrane of gram-negative bacteria (24). These complex macromolecules exhibit a variety of toxic and proinflammatory activities that are associated with the lipid A moiety and are causally related to the pathogenesis of gram-negative sepsis and septic shock (17, 18).Many of the local and systemic pathophysiologic phenomena produced by LPS in the exposed host result from the ability of LPS to activate host inflammatory cells (7), including monocytes, macrophages, and polymorphonuclear leukocytes. Recent attention has focused on putative LPS receptors found on the surfaces of these cells, the relation of these receptors to LPS-induced signal transduction, and the role of each in the development of proinflammatory responses.Membrane-bound CD14 (mCD14), a glycosyl phosphatidylinositol-anchored protein expressed on myeloid cells, is the best characterized LPS receptor identified to date (9, 33, 37). mCD14 appears to be part of a multicomponent LPS receptor functionally linked to the initiation of intracellular signaling events related to LPS-induced cell activation (29). The signaling unit of the LPS receptor is comprised of members of the Toll-like receptor family of transmembrane proteins characterized by their amphiphilic properties and leucine-rich repeats (31, 36). Serum-associated LPS-binding protein (LBP), which forms complexes with LPS through high-affinity attachment to the lipid A moiety, catalyzes LPS recognition by mCD14, resulting in the generation of LPS-induced proinflammatory signals (12,14).Recent experiments have attempted to define the roles of mCD14 and LBP in LPS-related septic events as well as the possible protective or therapeutic activities of proteins, including antibodies, that neutralize LPS by interrupting its proinflammatory interactions with mCD14 and LBP.We previously showed that LPS-specific monoclonal antibodies (MAbs) are capable of neutralizing cytokine-and transcription factor-inducing activities of LPS by inhibiting the binding of LPS to mCD14 expressed on human peripheral blood monocytes (PBMC) and on CD14-transfected Chinese hamster ovary fibroblasts (CHO-CD14 cells) (20,21).Polymyxin B (PMB), a cationic, cyclic peptide antibiotic, inhibits biological activities of LPS through high-affinity bind-* Corresponding author. Mailing address: