  We have studied the radiative corrections from the fourth generation leptons
in the context of the see-saw mechanism. We have estimated numerically the
differential cross section for the process (${\rm e}^{+}{\rm e}^{-}\!
\rightarrow {\rm W}^{+}{\rm W}^{-}$) at one-loop level, and found in the cross
section the threshold behaviors for Majorana neutrino productions.
