  The possibility of anisotropies in the speed of light relative to the
limiting speed of electrons is considered. The absence of sidereal variations
in the energy of Compton-edge photons at the ESRF's GRAAL facility constrains
such anisotropies representing the first non-threshold collision-kinematics
study of Lorentz violation. When interpreted within the minimal Standard-Model
Extension, this result yields the two-sided limit of 1.6 x 10^{-14} at 95%
confidence level on a combination of the parity-violating photon and electron
coefficients kappa_{o+} and c. This new constraint provides an improvement over
previous bounds by one order of magnitude.
