  We analyse the high-energy behavior of tree-level graviton Compton amplitudes
for particles of mass m and arbitrary spin, concentrating on a combination of
forward amplitudes that will be unaffected by eventual cross- couplings to
other, higher spins. We first show that for any spin larger than 2, tree-level
unitarity is already violated at energies well below the Planck scale M, if m
<< M. We then restore unitarity to this amplitude up to M by adding non-minimal
couplings that depend on the curvature and its derivatives, and modify the
minimal description - including particle gravitational quadrupole moments - at
scales O(1/m).
