We report an analysis of the nuclear dependence of the yield of Drell-Yan d
imuons from the 800 GeV/c proton bombardment of H-2, C, Ca, Fe, and W targe
ts. 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 l
oss and shadowing on the nuclear-dependence ratios versus the incident prot
on's momentum fraction and dimuon effective mass. The resulting energy loss
per unit path length is -dE/dz = 2.32 +/- 0.52 +/- 0.5 GeV/fm. This is the
first observation of a nonzero energy loss of partons traveling in a nucle
ar environment.