We analyze the phenomenological implications for new electroweak physi
cs in the Higgs sector in the framework of SU(2)L x U(1)Y theories tha
t naturally predict heavy Majorana neutrinos. We calculate the one-loo
p Majorana-neutrino contributions to the decay rates of the Higgs boso
n into pairs of quarks and intermediate bosons and to its production c
ross section via bremsstrahlung in e+e- collisions. It turns out that
these are extremely small in three-generation models. On the other han
d, 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.