  We develop the full finite lepton mass formalism for the production of real
photons via the Bethe-Heitler reaction of unpolarized leptons off unpolarized
nucleons. Genuine lepton mass effects are described, in particular their
dependence upon the lepton mass and the initial beam energy, as well as their
sensitivity to the nucleon isospin. In the minimum momentum transfer region,
these effects dominate the muon induced proton cross section and become
significant for electron scattering at small $x_B$.
