Unitarity of the Standard Model at the Higgs' Resonance

We compute a unitarity bound for higgs mass using one--loop corrected s--wave partial amplitude for $Z_L Z_L \rightarrow Z_L Z_L$ scattering. We use the equivalence theorem and show that the higgs mass has to be less than $\approx 600$ GeV in order to save the (perturbative) unitarity in the higgs' resonance region. We also discuss about the validity of perturbation expansion in the symmetry breaking sector.