We consider Dirac neutrino propagation in a medium with a randomly flu
ctuating-magnetic field by averaging over the fluctuations and derivin
g a simple kinetic equation for the neutrino spin rotation. We show th
at the rotation frequency depends on the size of the domains inside wh
ich the random magnetic field can be replaced by the mean field. When
the size L of the domains is test than the neutrino spin oscillation l
ength, or if L less than or similar to 10(-2)l(H)(T/MeV)-3 with l(H) t
he horizon length, the probability for neutrino helicity change will b
e suppressed.