  The effect of the relativistic spin rotation on two--particle spin states,
conditioned by the setting of the spins of the particles in their rest frames
and by the noncommutativity of the Lorentz transformations along noncolinear
directions, is discussed. Particularly, the transition from the c.m.s. of two
spin-1/2 particles to the laboratory is considered. When the vectors of the
c.m.s. particle velocities are not colinear with the velocity vector of the
c.m.s., the angles of the relativistic spin rotation for the two particles are
different. As a result, the relative fractions of the singlet and triplet
states in the relativistic system of two spin-1/2 particles depend on the
concrete frame in which the two-particle system is analyzed.
