  The existence of extra chiral generations with all fermions heavier than
$m_Z$ is strongly disfavored by the precision electroweak data. However the
data still allow a few extra generations if the neutral leptons have masses
close to 50 GeV. Such heavy neutrino can be searched in the reaction $e^+ e^-
\to N\bar{N}\gamma$. Existence of 50 GeV neutrinos makes standard model Higgs
boson invisible.
