  We study the evolution of massive mixed Dirac and Majorana neutrinos in
matter under the influence of a transversal magnetic field. The analysis is
based on relativistic quantum mechanics. We solve exactly the evolution
equation for relativistic neutrinos, find the neutrino wave functions, and
calculate the transition probability for spin-flavor oscillations. We analyze
the dependence of the transition probability on the external fields and compare
the cases of Dirac and Majorana neutrinos. The evolution of Majorana particles
in vacuum is also studied and correction terms to the standard oscillation
formula are derived and discussed. As a possible application of our results we
discuss the spin-flavor transitions in supernovae.
