  The assumption that two-loop top corrections are well approximated by the
$O(G_mu^2 mt^4)$ contribution is investigated. It is shown that in the case of
the ratio neutral-to-charged current amplitudes at zero momentum transfer the
$O(G_mu^2 mt^2 M_Z^2)$ terms are numerically comparable to the $m_t^4$
contribution for realistic values of the top mass. An estimate of the
theoretical error due to unknown two-loop top effect is presented for a few
observables of LEP interest.
