The ability of membrane filters to adsorb virus efficiently afforded a rapid method to estimate the specificity of radioactive labeling of virus-nuclide that was not virus associated was found in the filtrate (Herrmann and Cliver 1973c) . We also devised an electrophoretic method for collecting viral particles on a polycarbonate membrane so that the particles could be enumerated by scanning electron microscopy (Heinz et al. 1986 ).