  We obtain a relativistically covariant wave equation for neutrinos in dense
matter and electromagnetic field, which describes both flavor oscillations and
neutrino spin rotation. Using this equation we construct a quasi-classical
theory of these phenomena. We obtain the probabilities of arbitrary spin-flavor
transitions assuming the external conditions to be constant. We demonstrate
that the resonance behavior of the transition probabilities is possible only
when the neutrino flavor states cannot be described as superpositions of the
mass eigenstates. We discover that a resonance, which is similar to the
Mikheev-Smirnov-Wolfenstein resonance, takes place for neutrinos in magnetic
field due to the transition magnetic moments. This resonance gives an
opportunity to determine, whether neutrinos are Dirac or Majorana particles.
