  The tree-level amplitude for the scattering of two gauge particles
constrained to move on the two distinct boundaries of eleven-dimensional
space-time in the Horava-Witten formulation of M-theory is constructed. At low
momenta this reproduces the corresponding tree-level scattering amplitude of
the E_8xE_8 heterotic string theory. After compactification to nine dimensions
on a large circle with a suitable Wilson line to break the symmetry to
SO(16)xSO(16) this amplitude is used to describe the scattering of two massive
SO(16) spinor states - one from each factor of the unbroken symmetry group. The
amplitude contains a component that is associated with the exchange of a
Kaluza-Klein charge between the boundaries, which is interpreted as the
exchange of a D-particle between orientifold planes in the Type IA theory. This
is related by T-duality to the effect of a non-BPS D-instanton in the Type I
theory which is only invariant under those elements of O(16)xSO(16) that are in
SO(16)xSO(16).
