  Considering two spinless particles, a simple, approximate boost rule is
derived that is sufficient to keep the mass invariant and to relate
interactions, vertex functions, wave functions and t-matrices of the instant
two-body problem in an arbitrary frame to those in the c.m. frame. The boost
generator ${\bf K}$ that is used provides a solution to the Poincar\'{e}
algebra except for the commutator between two components of ${\bf K}$, for
which it provids an approximate solution.
