  We consider active-sterile neutrino conversions in the early universe hot
plasma in the presence of a random magnetic field generated at the electroweak
phase transition. Within a random field domain the magnetization asymmetry of
the lepton antilepton plasma produced by a uniform constant magnetic field is
huge in contrast to their small density asymmetry, leading to a drastic change
in the active-sterile conversion rates. Assuming that the random field provides
the seed for the galactic field one can estimate the restrictions from
primordial nucleosynthesis. Requiring that the extra sterile \neu does not
enter in equilibrium with the active ones before nucleosynthesis we find limits
of the oscillation parameters which are stronger than in the isotropic case.
