  We investigate the theory of particles with arbitrary spin and magnetic
moment in the Lorentz representation (0,s)X(s,0) in an external constant and
uniform electromagnetic field. We obtain the density matrix of free particles
in pure spin states. The differential probability of pair producing particles
with arbitrary spin by an external constant and uniform electromagnetic field
is found using the exact solutions. We calculate the imaginary and real parts
of the Lagrangian in an electromagnetic field that takes into account the
vacuum polarization.
