  $CP$-violating asymmetries calculated from the Breit-Wigner approximation for
unstable particle propagators violate $CPT$. A formalism satisfying $CP$
invariance and unitarity is presented. Applications are given to $t$ decays.
For the decay $t\to b\nu_{\tau}\bar{\tau}$ in a class of $CP$-violating models
the $CP$-violating asymmetry turns out to be $10^{-4}$ times smaller than the
previously reported results.
