Vibrational sum frequency generation (VSFG) spectroscopy was applied to investigate interaction between an antibiotic, polymyxin B (PMB), and lipid monolayers at the air/water interface. The VSFG spectra in the CH stretching region observed for the monolayer of the zwitterionic (or neutral) dipalmitoylphosphatidylcholine (DPPC) spread on pure water and for the monolayer spread on a PMB aqueous solution (10(-6) M) are similar to each other, indicating the absence of interaction between the DPPC monolayer and PMB. On the other hand, the VSFG spectra of the monolayer of the anionic (or negatively charged) dipalmitoylphosphatidylglycerol (DPPG) spread on the PMB solution give rise to a prominent peak at 2850 cm(-1) due to a CH2 asymmetric stretching mode of the alkyl group, which is absent in the spectra of the DPPG monolayer on pure water, indicating that the binding of PMB to the DPPG monolayer induces the gauche defect of the alkyl group. The result suggests that the gauche defect caused by the specific binding of PMB to negatively charged parts of the cell membrane causes the increase in the fluidity and/or the permeability of the cell membrane, one of the key factors in the antibiotic activity of PMB. The VSFG spectra in the OH stretching region observed for the DPPC and DPPG monolayers on pure water are dominated by a broad band centered around 3200 cm(-1), indicating the presence of an ordered array of hydrogen-bonded water molecules associated with the hydrophilic headgroups of the monolayers. The VSFG spectra in the OH stretching region observed for the DPPC monolayer spread on the PMB aqueous solution (10(-6) M) are more or less similar to those observed for the monolayer on pure water. In contrast, the VSFG spectra of the DPPG monolayer on the PMB solution give only a very weak or almost indiscernible band near 3200 cm(-1). The binding of PMB to the interfacial region of the DPPG monolayer cancels the electrostatic field, prohibiting the formation of the ordered array of the water molecules in the interfacial region.