  Low-energy high-resolution neutrino-electron scattering experiments may play
an important role in testing the gauge structure of the electroweak
interaction. We discuss the use of strong radioactive neutrino sources (e.g.
$^{51}$Cr) in underground experiments such as BOREXINO, HELLAZ and LAMA. We
display the sensitivity of these detectors in testing the possible existence of
extra neutral $E_6$ gauge bosons.
