  We investigate the linear stability properties of the plane interface
separating two relativistic magnetized flows in relative motion. The two flows
are governed by the (special) relativistic equations for a magnetized perfect
gas in the infinite conductivity approximation. By adopting the vortex-sheet
approximation, the relativistic magnetohydrodynamics equations are linearized
around the equilibrium state and the corresponding dispersion relation is
derived and discussed. The behavior of the configuration and the regimes of
instability are investigated following the effects of four physical parameters,
namely: the flow velocity, the relativistic and Alfv\'enic Mach numbers and the
inclination of the wave vector on the plane of the interface. From the
numerical solution of the dispersion relation, we find in general two separate
regions of instability, associated respectively with the slow and fast
magnetosonic modes. Modes parallel to the flow velocity are destabilized only
for sufficiently low magnetization. For the latter case, stabilization is
attained, additionally, at sufficiently large relativistic velocities between
the two flows in relative motion. The relevance of these results to the study
of the stability of astrophysical jets is briefly commented.
