  Complete one-loop electroweak radiative corrections to the cross section of
the process $e^+e^- \to \mu^-\mu^+ (\tau^-\tau^+)$ are evaluated with the help
of the \SANC system. Higher-order contributions of the initial state radiation
are computed in the QED structure function formalism. Numerical results are
given for the center-of-mass energy range $\sqrt{s}=5, ~7$~GeV for various
polarization degrees of the initial particles.
