Аннотация:We obtain a wave equation for neutrino in dense matter and electromagnetic field considering neutrinos with different masses as the states of a SU(3)neutrino multiplet. In quasi-classical approximation we obtain explicit formulasfor spin-flavor transition probabilities in the two-flavor model in the case of movingunpolarized matter with constant density and velocity and in the case of constantelectromagnetic field taking into account the neutrino transition magnetic moment.In the latter case a new resonance arises, which is an analog of Mikheev-Smirnov-Wolfenstein resonance.