  We study the $\gamma$-Z interference in the process $e^+ e^-\to \Upsilon \to
\tau^+ \tau^-$ as a means to measure the neutral current coupling of the
b-quark. The helicity amplitudes are calculated from resonant and background
diagrams and the spin density matrix of the final state is discussed. The spin
analyzer of the $\tau$'s is illustrated with the decays $\pi \nu$ and $\rho \nu
\to (\pi \pi) \nu$. With $10^8 \Upsilon$ a sensitivity to $g_V^b$ of a few per
cent could be reachable.
