  We calculate the typical size of loop corrections to electroweak observables
arising from non-standard $Z {\overline t } t$ and $W t b$ vertices. We use an
effective Lagrangian formalism based on the electroweak gauge group
$SU(2)_L\times U(1)_Y \rightarrow U(1)_{EM}$. Limits on the non-standard model
top quark couplings from electroweak observables are presented and compared
with previously obtained limits.
