For the kinematical region when a hard photon is emitted predominantly close to the direction of motion of a longitudinally polarized initial electron and relatively small momentum is transferred to a proton we calculate the azimuthal asymmetry of photon emission. It arises from the interference of the Bethe-Heitler amplitude and amplitudes described by a heavy photon impact factor. Azimuthal asymmetry does not decrease in the limit of infinite c.m.s. energy. The lowest order expression for the impact factor of a heavy photon is presented. ©2001 The American Physical Society.