  We report an analysis of the nuclear dependence of the yield of Drell-Yan
dimuons from the 800 GeV/c proton bombardment of $^2H$, C, Ca, Fe, and W
targets. Employing a new formulation of the Drell-Yan process in the rest frame
of the nucleus, this analysis examines the effect of initial-state energy loss
and shadowing on the nuclear-dependence ratios versus the incident proton's
momentum fraction and dimuon effective mass. The resulting energy loss per unit
path length is $-dE/dz = 2.32 \pm 0.52\pm 0.5$ GeV/fm. This is the first
observation of a nonzero energy loss of partons traveling in nuclear
environment.
