Membrane filters made by the evaporation of solvents from mixtures of cellulose nitrate (Gradocol) or cellulose acetate (Millipore, Oxoid), ether and ethanol. By variations of the solvent proportions and the evaporative procedure, permeable filters of any defined size from 3 μm down to 10nm are obtained so reliably that the filters may be used to determine virus sizes.