  The Glashow resonance is a rapid enhancement of the cross section for
scattering of electron antineutrinos on electrons at the $W^-$ boson production
threshold due to the $s$-channel contribution. This resonance is being searched
for at large volume neutrino detectors in which the reaction
$\bar\nu_ee^-\rightarrow W^-$ to be initiated by cosmic ray antineutrinos of
energies about 6.3 PeV. The relatively small neutrino flux reaching the Earth
together with the necessity of analyzing ultra-high-energy final states make
such searches challenging. We argue here that the Glashow resonance may
contribute to the process $e^+e^-\rightarrow~W^+W^-$. By extending the method
of the effective (equivalent) particles to the neutral leptons, we relate the
distribution of the effective neutrinos in the electron to the total cross
section for $e^+e^-\rightarrow~W^+W^-$. Our approach gives a good fit to
existing experimental data measured at the collider LEP at CERN and allows one
to interpret these measurements as an observation of the Glashow resonance. We
also discuss an advantage of future electron-positron colliders for probing the
resonance.
