  In the standard model, the requirements of vacuum stability and the validity
of perturbation theory up to the unification scale force the mass of the Higgs
boson to be approximately between 130 GeV and 180 GeV. We re-examine these
requirements in the (non-supersymmetric) two-Higgs doublet model, in the light
of the large top quark mass, and constrain the masses of the Higgs bosons in
this model. It is found that the mass of the charged Higgs boson must be
lighter than 150 GeV. This bound is below the lower bound in the popular
model-II two-Higgs doublet model, and thus we conclude that this model cannot
be valid up to the unification scale. The bounds on the neutral Higgs scalars
are also discussed.
