  A semi-empirical calculation of the 6s - 7s Stark amplitude $\alpha$ in Cs
has been performed using the most accurate measurements and calculations of the
electromagnetic amplitudes available. This is then used to extract the
parameters of the electroweak theory from experimental data. The results are:
$\alpha = 269.0 (1.3) a_0^3$, weak charge of Cs $Q_W = -72.41(25)_{exp}
(80)_{theor}$, deviation from the Standard model $S = -1.0(.3)_{exp}
(1.0)_{theor}$ and limit on the mass of the extra Z-boson in SO(10) model
$M_{Z_x} > 550 GeV$.
