Background. Bactericidal permeability increasing protein (BPI) binds to and neutralizes lipopolysaccharide (LPS, endotoxin). Small synthetic peptides based on the amino acid sequence of the LPS binding domain of BPI neutralize LPS, albeit inefficiently. Although the LPS binding domain of native BPI possesses a β-turn secondary structure, this structure is not present in small derivative peptides. The purpose of this study was to determine whether the addition of a β-turn to a BPI-derived peptide is associated with more potent endotoxin antagonism. Methods. We generated a hybrid peptide (BU3) on the basis of (1) a portion of the LPS binding domain from BPI and (2) amino acids known to initiate a β-turn. BU3 folds with a β-turn, and we tested its effects on LPS neutralization and LPS-induced tumor necrosis factor-α secretion, comparing it with BPI-derived peptide BG22 that lacks a β-turn and to an irrelevant peptide (BG16). Results. Compared with BG22, BU3 demonstrated enhanced LPS neutralization and inhibition of LPS-induced tumor necrosis factor-α secretion in vitro and a similar diminution of endotoxemia and tumor necrosis factor-α secretion in a murine model of endotoxemia. Conclusions. These data demonstrate the potential for enhancing the biologic activity of a BPI-derived peptide endotoxin antagonist via manipulation of its conformational structure.