Neutralization of the Limulus activities of lipid A (a) and LPS (b) by the peptide sequences containing amino acid residues 20-31 to 17-31. (a) The peptides (25 ng/ml) of amino acid residues 20-31 to 17-31 were incubated with diphosphoryl and monophosphoryl lipid A, and the remaining Limulus activity was analyzed (expressed as percentage of maximal LAL activity). The experiment was repeated three times, except for peptide sequences 17-31 and 20-31 (single run for diphosphoryl lipid A; repeated once for monophosphoryl lipid A). *, P > 0.05; **, P > 0.01 (Student's t test). In panel b, the remaining activity of LPS O111 and O55 after incubation with 5 μg/ml of peptide sequence 19-31 (HL9), polymyxin B, and BPI peptide is shown. The experiment was repeated once except for O111 LPS (single run).