  We study the Fundamental Plane of field early-type galaxies at intermediate
redshift, using HST WFPC2 observations and deep Keck spectroscopy. Structural
parameters and internal velocity dispersions are measured for eighteen galaxies
at 0.15 < z < 0.55. Rest frame M/L_B ratios are determined from the Fundamental
Plane and compared to those of cluster early-type galaxies at the same
redshifts. The systematic offset between M/L ratios of field and cluster
early-type galaxies at intermediate redshift is small, and not significant: <ln
M/L_B>(field) - <ln M/L_B>(cluster) = -0.18 +- 0.11. The M/L ratios of field
early-type galaxies evolve as \Delta (ln M/L_B) = (-1.35 +- 0.35) z, very
similar to cluster early-type galaxies. After correcting for luminosity
evolution, the FP of field early-type galaxies has a scatter 0.09 +- 0.02 in
log r_e, similar to that in local clusters. The scatter appears to be driven by
low mass S0 galaxies; for the elliptical galaxies alone we find 0.03 +- 0.03.
There is a hint that the FP has a different slope than in clusters, but more
data are needed to confirm this. The similarity of the M/L ratios of cluster
and field early-type galaxies provides a constraint on the relative ages of
their stars. At <z> = 0.43, field early-type galaxies are younger than cluster
early-type galaxies by only 21 +- 13 %, and we infer that the stars in field
early-type galaxies probably formed at z >~ 1.5. Recent semi-analytical models
for galaxy formation in a Lambda-CDM Universe predict a systematic difference
between field and cluster galaxies of \Delta (ln M/L_B) ~ -0.6, much larger
than the observed difference. This result is consistent with the hypothesis
that field early-type galaxies formed earlier than predicted by these models.
