  We present a new preliminary measurement of the parity-violation parameter Ab
using a self-calibrating vertex-charge technique. In the SLD experiment we
observe hadronic decays of polarized Z bosons produced in collisions between
longitudinally polarized electrons and unpolarized positrons at the SLAC Linear
Collider. A sample of bb events is selected using the topologically
reconstructed mass of B hadrons. From out 1996-8 data sample of approximately
400,000 hadronic Z decays, we obtain Ab = 0.897 +/- 0.027 (stat) +0.036/-0.034
(syst).
