  This talk is based on the article hep-ph/9903517 written in collaboration
with Sven Bergmann and Yuval Grossman. An analysis of the effective potential
for neutrino propagation in matter, assuming a generic set of Lorentz invariant
non-derivative interactions is presented. In addition to vector and axial
vector couplings, also tensor interactions can give coherent effects if the
medium is polarized, and the components of a tensor potential transverse to the
direction of neutrino propagation can induce a neutrino spin-flip.
