  In view of the recent and future electroweak precision data accumulated at
LEP and SLC, we systematically analyze possible new physics effects that may
occur in the leptonic sector within the context of $SU(2)_R \times SU(2)_L
\times U(1)_{B-L}$ theories. It is shown that nonobservation of
flavour-violating $Z$-boson decays, lepton universality in the decays $Z\to
l\bar{l}$, and universality of lepton asymmetries at the $Z$ peak form a set of
complementary observables, yielding severe constraints on the parameter space
of these theories. Contributions of new-physics effects to $R_b=\Gamma(Z\to
b\bar{b})/\Gamma(Z\to \mbox{hadrons})$ are found to give interesting mass
relations for the flavour-changing Higgs scalars present in these models.
