  We review the information about a potentially strong non-standard
four-neutrino interaction that can be obtained from available experimental
data. By using LEP results and nucleosynthesis data we find that a contact
four-fermion neutrino interaction that involve only left-handed neutrinos or
both left-handed and right-handed neutrinos cannot be stronger than the
standard weak interactions. A much stronger interaction involving only
right-handed neutrinos is still allowed.
