  We show that the observation of energetic charged leptons from semileptonic
$b$ becays at the LEP and SLC $Z$--boson factories offers a unique opportunity
to measure the quark mixing matrix element $V_{ub}$. We present various
distributions of the $b$ decay products to show that the $b \to u$ decays can
be cleanly separated from the $b \to c$ decays, with a signal of $O(100)$
events per million $Z$ bosons produced.
