The dependence of the interfacial tension of a lipid bilayer on the pH of the aqueous solution has been studied. A theoretical equation is derived to describe this dependence. Interfacial tension measurements of an egg phosphatidylcholine bilayer were carried out. The experimental results agreed with those derived from the theoretical equation obtained close to the isoelectric point within a range of three pH units. A maximum corresponding to the isoelectric point appears both in the theoretical equation and in the experimental data.