  The influence of the nuclear medium on the production of charged hadrons in
semi-inclusive deep inelastic scattering has been studied by the HERMES
experiment at DESY using 27.5 GeV positrons.
  A substantial reduction of the multiplicity of charged hadrons and identified
charged pions from nuclei relative to that from deuterium has been measured as
function of the relevant kinematic variables. The preliminary results on
krypton show a larger reduction of the multiplicity ratio $R_M^{h}$ with
respect to the one previously measured on nitrogen and suggest a possible
modification of the quark fragmentation process in the nuclear environment.
