  Non-perturbative corrections to the LO cross section formulae for DIS and the
Drell-Yan process were calculated. The interaction of partons was taken into
account via dressing the incoming quark lines with spectral functions. We found
the effect of ground state correlation in DIS to be large in the region of
small Bjorken x and low momentum transfer Q. For a quark width of the order of
200 MeV, the deviation from parton model cross section reaches as much as 50%
for Q^2=10 GeV^2. On the other hand, for the values of Q well above the
resonance region, the effect of ISI turned out to be small in DIS, but still
substantial for the triple differential Drell-Yan cross section. Our
calculations show that the shape of the high mass Drell-Yan pair transverse
momentum distribution is fully determined by quark off-shellness. From
comparison to the resent data on the Drell-Yan cross section from NuSea
collaboration, we obtained for the quark width in the nucleon the value about
200 MeV.
