Endotoxin [lipopolysaccharide (LPS)], the major antigen of the outer membrane of Gram-negative bacteria, consists of a variable-size carbohydrate chain that is covalently linked to N,O-acylated beta-1,6-D-glucosamine disaccharide 1,4'-bisphosphate (lipid A). The toxic activity of LPS resides in the lipid A structure. The structural features of synthetic peptides that bind to lipid A with high affinity, detoxify LPS in vitro, and prevent LPS-induced cytokine release and lethality in vivo were defined. The binding thermodynamics were comparable to that of an antigen-antibody reaction. Such synthetic peptides may provide a strategy for prophylaxis and treatment of LPS-mediated diseases.