  We propose a new criterium for saturation of the density of partons both in
nucleons and nuclei. It is applicable to any multiple scattering model which
would be used to compute the number of strings exchanged in $ep$ and $eA$
collisions. The criterium is based on percolation of strings, and the onset of
percolation is estimated from expectations coming from the study of heavy ion
collisions at high energies. We interpret this onset as an indication of
saturation of the density of partons in the wave function of the hadron. In
order to produce quantitative results, a particular model fitted to describe
present HERA data and generalized to the nuclear case is used. Nevertheless,
with the number of scatterings controlled by the relation between inclusive and
diffractive processes, conclusions are weakly model-dependent as long as
different models are tuned to describe the experimental data. This constitutes
a new approach, based on the eikonal description of soft hadronic collisions,
and different from others which employ either perturbative QCD ideas or
semiclassical methods. It offers an alternative picture for saturation in the
small $Q^2$ region.
