  We construct a spectral representation of neutrino propagator in moving
matter or in external magnetic field. In both cases there exists fixed
four-dimensional axis of polarization, such that the corresponding spin
projectors commute with propagator. As a result, all eigenvalues of propagator
and, consequently, dispersion laws for neutrino in media are classified
according to spin projection onto this axis. Use of the found spin projectors
simplifies essentially the eigenvalue problem and allows to build spectral
representation of propagator in moving matter or external magnetic field in
analogy with the vacuum propagator.
