  We report an analysis of the nuclear dependence of the yield of Drell-Yan
(DY) dimuons from the 800 GeV/c proton bombardment of $^2H$, C, Ca, Fe, and W
targets. A light-cone formulation of the DY process is employed in the rest
frame of the nucleus. In this frame, for $x_2\ll x_1$, DY production appears as
bremsstrahlung of a virtual photon followed by decay into dileptons. We treat
the two sources of nuclear suppression, energy loss and shadowing, in a
consistent formulation. Shadowing, involving no free parameters, is calculated
within the light-cone dipole formalism. Initial-state energy loss, the only
unknown in the problem, is determined from a fit to the nuclear-dependence
ratio versus $x_1$. With the assumption of constant energy loss per unit path
length, we find $-dE/dz = 2.32 \pm 0.52\pm 0.5$ GeV/fm. This is the first
observation of a nonzero energy loss of partons traveling in nuclear
environment.
