  The joining--splitting interaction of non-critical bosonic string is analyzed
in the light-cone formulation. The Mandelstam method of constructing tree
string amplitudes is extended to the bosonic massive string models of the
discrete series. The general properties of the Liouville longitudinal
excitations which are necessary and sufficient for the Lorentz covariance of
the light-cone amplitudes are derived. The results suggest that the covariant
and the light-cone approach are equivalent also in the non-critical dimensions.
Some aspects of unitarity of interacting non-critical massive string theory are
discussed.
