  In this paper we develop an algebraic technique for building relativistic
models in the framework of direct-interaction theories. The interacting mass
operator M in the Bakamjian-Thomas construction is related to a quadratic
Casimir operator C of a non-compact group G. As a consequence, the S matrix can
be gained from an intertwining relation between Weyl-equivalent representations
of G. The method is illustrated by explicit application to a model with SO(3,1)
dynamical symmetry.
