The renormalization of general theories with inter-family mixing of Di
rac and/or Majorana fermions is studied at the one-loop electroweak or
der, The phenomenological significance of the mixing-matrix renormaliz
ation is discussed, within the context of models based on the SU(2)(L)
xU(1)(Y) gauge group, The effect of radiative neutrino masses present
in these models is naturally taken into account in this formulation. A
s an example, charged-lepton universality in pion decays is investigat
ed in the heavy-neutrino limit. Non-decoupling heavy-neutrino effects
induced by mixing renormalization are found to considerably affect the
predictions in these new-physics scenarios.