The Langmuir-Blodgett films of purple membrane (PM) immobilized on a gold electrode produced steady cathodic photocurrents under cathodic polarization of the electrode in aqueous electrolyte. The photocurrent is attributable to proton-reducing current that responds to proton release from light-excited bacteriorhodopsin. Effect of DC electric field at the water-air interface on the orientation of PM monolayer film was investigated.