  We report a new measurement of A_b using data obtained by SLD in 1997-98.
This measurement uses a vertex tag technique, where the selection of a b
hemisphere is based on the reconstructed mass of the bottom hadron decay
vertex. The method uses the 3D vertexing capabilities of SLD's CCD vertex
detector and the small and stable SLC beams to obtain a high b-event tagging
efficiency and purity of 78% and 97%, respectively. Charged kaons identified by
the CRID detector provide an efficient quark-antiquark tag, with the analyzing
power calibrated from the data. We obtain a preliminary result of A_b = 0.997
\pm 0.044 \pm 0.067
