  The vacuum stability lower bound on the mass of the Higgs boson is
numerically investigated in an $SU(2)_L \otimes SU(2)_R$ symmetric Yukawa
model, which describes two heavy degenerate fermion doublets in the limit of
vanishing gauge couplings. Good agreement with perturbation theory is found,
although the couplings are strong. The upper bound on the fermion mass and
renormalized Yukawa coupling is also determined in the part of bare parameter
space where reflection positivity has been proven.
