Polyethylene glycol (PEG) and dextran were covalently coupled, or only adsorbed, to the surface of three kinds of inorganic particles in order to shield their surface and reduce their nonspecific binding to red blood cells. Surface modification as well as interaction of particles with red blood cells was followed up by particle electrophoresis. This allowed a quick evaluation of the efficiency of polymer coupling. Moreover, the nonspecific binding of particles to red blood cells was easily investigated with cell electrophoresis, showing the inhibitory effect of immobilized PEG-5000 or dextran. The electrophoretic mobility analysis presented here may be used for screening blood compatibility of particulate drug carriers and could be helpful in formulating long-living circulating particles.