  We show that it is possible to define a Lorentz-covariant reduced spin
density matrix for massive particles. Such a matrix allows one to calculate the
mean values of observables connected with spin measurements (average
polarizations). Moreover, it contains not only information about polarization
of the particle but also information about its average kinematical state. We
also use our formalism to calculate the correlation function in the
Einstein--Podolsky--Rosen--Bohm type experiment with massive relativistic
particles.
