[en] A previously developed procedure is used for generating the multiple operators of the weak and electromagnetic nuclear current from single-nucleon matrix elements. The analysis is extended through O((γ/c)²sub(nucleon))by including relativistic corrections in the matrix elements. The author neglects explicit exchange currents and treats ambiguities arising from the off-mass-shell behavior of the nucleons in a consistent fashion using two alternative methods of calculation. One method equates the energy transfer to the energy difference between single-nucleon states, the other uses the lepton energy loss; the results agree with similar calculations done previously. The author also proves a theorem indicating the circumstances under which these ambiguities are absent. In explicit calculations in the A=6 and A=12 systems using previously determined nuclear parameters, it is found that second-order corrections are small when second-class currents are absent. When second-class currents are included, their effects on γ-decay are ambiguous; however in processes involving large momentum transfers (e.g. neutrino scattering), their effects may be large and are insensitive to the treatment of the off-shell behavior of the nucleons. (Auth.)