  We constrain the post-Newtonian gravity parameter gamma on kiloparsec scales
by comparing the masses of 15 elliptical lensing galaxies from the Sloan Lens
ACS Survey as determined in two independent ways. The first method assumes only
that Newtonian gravity is correct and is independent of gamma, while the second
uses gravitational lensing which depends on gamma. More specifically, we
combine Einstein radii and radial surface-brightness gradient measurements of
the lens galaxies with empirical distributions for the mass concentration and
velocity anisotropy of elliptical galaxies in the local universe to predict
gamma-dependent probability distributions for the lens-galaxy velocity
dispersions. By comparing with observed velocity dispersions, we derive a
maximum-likelihood value of gamma = 0.98 +/- 0.07 (68% confidence). This result
is in excellent agreement with the prediction of general relativity that has
previously been verified to this accuracy only on solar-system length scales.

Astrophysics;General Relativity and Quantum Cosmology