Cellulose nitrate membranes were used as one of the adsorbents in concentrating viruses from water. For adsorption to occur, salts were required. With increase in valency of salt, less salt was necessary for enhanced virus adsorption to membranes. Trivalent salts were more effective because they could be used at only 1% the concentration required for divalent salts. Thus, 0.5 mM AlCl3 was as effective as 50 mM MgCl2. For testing 500 gal of water, only 0.24 kg of AlCl3 was required in contrast to 20 kg of MgCl2. Virus could then be eluted from such membranes, having an area of 486 cm2, with 250 ml of pH 11.5 buffer. Lowering the pH of the eluate and adding AlCl3 permitted the virus to be quickly readsorbed on a smaller cellulose membrane, i.e., 4 cm2. Virus for assay was eluted from the small membrane in 1 ml. This procedure has provided the basis for concentrating minute amounts of virus from large volumes of water.