  We consider extensions of the Standard Model with an extra U(1) gauge boson
which couples to B-(alpha L_e + beta L_mu + gamma L_tau) with alpha + beta +
gamma = 3. We show that the extra gauge boson necessarily mixes with the Z
leading to potentially significant corrections to the Z-f-fbar vertex. The
constraints on the size of this correction imposed by the Z-pole data from LEP
and SLD are derived.
