The measurements carried out by the electric field probe onboard the PHOBOS-2 spacecraft reveal that the mass-loading taking place in the solar wind far upstream of the Martian bow shock is dependent upon the orientation of the interplanetary magnetic field. The asymmetry of mass-loading is caused by H+ pickup ions originating from the extended cold hydrogen Martian exosphere and reflected from the bow shock. The distribution of pickup protons in the upstream region is analyzed in a test particle approach, which seems to be confirmed by the Phobos data. ¿ American Geophysical Union 1994