  We suggest the use of a universality-breaking observable based on lepton
asymmetries as derived from the left-right asymmetry and the $\tau$
polarization at the $Z$ peak, which can efficiently constrain the parameter
space of unified theories. The new observable is complementary to the leptonic
partial width differences and it depends critically on the chirality of a
possible non-universal $Z$-boson coupling to like-flavour leptons. The LEP/SLC
potential of probing universality violation is discussed in representative
low-energy extensions of the Standard Model (SM) that could be derived by
supersymmetric grand unified theories, such as the SM with left-handed and/or
right-handed neutral isosinglets, the left-right symmetric model, and the
minimal supersymmetric SM.
