  We explore the properties of spin splitting for neutral particles possessing
electric and magnetic dipole moments propagating in an electromagnetic field.
Two notable features of the spin splitting and the associated Larmor precession
are found, which are consequences of special relativity. First, we report the
existence of an upper limit of spin splitting equal to twice the rest energy of
the particle, and a corresponding upper limit for the Larmor precession
frequency. Second, we predict the noninvariance of the spin splitting and the
corresponding Larmor frequency with respect to Lorentz boosts, which bears
resemblance to the classical Doppler effect.
