  The mass operator M is introduced as an independent dynamical variable which
is taken as the translation generator P_4 of the inhomogenous De Sitter group.
The classification of representations of the algebra P(1,4) of this group is
performed and the corresponding P(1,4) invariant equations for variable-mass
particles are written out. In this way we have succeeded, in particular, in
uniting the ``external'' and ``internal'' (SU_2) symmetries in a non-trivial
fashion.
