  The possibility of grand unification of the standard model (SM) with fermion
spectrum extended to include mirror fermions is examined. SM gauge couplings do
not automatically unify. SO(10) grand unification is studied with one
intermediate scale. Renormalization group equations (RGE) for fermion Yukawa
couplings and the scalar self-coupling are studied numerically at one and two
loop level. Strong restrictions for mirror fermion masses are obtained assuming
perturbative unification. Mirror masses much smaller than the tree unitarity
bounds are required. In particular mirror leptons have to be around 50 GeV.
Consistency of the mirror fermion model with LEP precision data is established.
A direct search for single production of mirror neutrinos at LEP could exclude
or confirm the GUT version of the mirror fermion model.
