  Results from several recent experiments provide inderect evidences in the
favor of existence of a 4th generation neutrino. Such a neutrino of mass about
50 GeV is compatible with current physical and astrophysical constraints and
well motivated in the framework of superstring phenomenology. If sufficiently
stable the existence of such a neutrino leads to the drastic change of Higgs
boson physics: for a wide range of Higgs boson masses the dominant mode of
Higgs boson decay is invisible and the branching ratios for the most promising
modes of Higgs boson search are significantly reduced. The proper strategy of
Higgs boson searches in such a framework is discussed. It is shown that in the
same framework the absence of a signal in the search for invisible Higgs boson
decay at LEP means either that the mass of Higgs is greater than 113.5 GeV or
that the mass difference between the Higgs mass and doubled neutrino mass is
small.
