  We analyze the phenomenological implications for new electroweak physics in
the Higgs sector in the framework of SU(2)_L x U(1)_Y theories that naturally
predict heavy Majorana neutrinos. We calculate the one-loop Majorana-neutrino
contributions to the decay rates of the Higgs boson into pairs of quarks and
intermediate bosons and to its production cross section via bremsstrahlung in
e^+e^- collisions. It turns out that these are extremely small in three-
generation models. On the other hand, the sizeable quantum corrections
generated by a conventional fourth generation with a Dirac neutrino may be
screened considerably in the presence of a Majorana degree of freedom.
