2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49271We 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.12 pages, 2 figures, latex with ws-p8-50x6-00.cls, Talk presented in "Neutrino Mixing", in honour of Samoil Bilenky's 70th Birthday, Torino, March 1999High Energy Physics - Phenomenologytext