A number of pathogens of the upper respiratory tract express an unusual prokaryotic structure, phosphorylcholine (ChoP), on their cell surface. We tested the hypothesis that ChoP, also found on host membrane lipids in the form of phosphatidylcholine, acts so as to decrease killing by antimicrobial peptides that target differences between bacterial and host membranes. In Haemophilus influenzae, ChoP is a phase-variable structure on the oligosaccharide portion of the lipopolysaccharide (LPS). There was a bactericidal effect of the peptide LL-37/hCAP18 on a nontypeableH. influenzae strain, with an increasing selection for the ChoP⁺ phase as the concentration of the peptide was raised from 0 to 10 μg/ml. Moreover, constitutive ChoP-expressing mutants of unrelated strains showed up to 1,000-fold-greater survival compared to mutants without ChoP. The effect of ChoP on resistance to killing by LL-37/hCAP18 was dependent on the salt concentration and was observed only when bacteria were grown in the presence of environmental choline, a requirement for the expression of ChoP on the LPS. Further studies established that there is transcription of the LL-37/hCAP18 gene on the epithelial surface of the human nasopharynx in situ and inducible transcription in epithelial cells derived from the upper airway. The presence of highly variable amounts of LL-37/hCAP18 in normal nasal secretions (<1.2 to >80 μg/ml) was demonstrated with an antibody against this peptide. It was concluded that ChoP alters the bacterial cell surface so as mimic host membrane lipids and decrease killing by LL-37/hCAP18, an antimicrobial peptide that may be expressed on the mucosal surface of the nasopharynx in bactericidal concentrations.