The association of poliovirus with membrane filters results from both electrostatic and hydrophobic interactions. At low pH, electrostatic interactions appear to dominate. However, at high pH, hydrophobic interactions appear to dominate with both Millipore and Zeta plus filters. With both filters, viral elution was prevented at high pH by the presence of antichaotropic salts, which strengthen hydrophobic associations. This effect was antagonized by detergents and by chaotropic salts, which weaken hydrophobic associations. Excellent correlation was observed between viral elution, the solubilization of adenine, and the micelle-formation process in the presence of chaotropic and antichaotropic agents. Such simple measurements of the relative ability of solvents to accommodate hydrophobic groups may be of predictive value in designing methods for concentration and purification of viruses. The hypothesis that hydrophobic interactions are the primary force involved in the stabilization of virus attachment at high pH is consistent with observations that cannot be explained by consideration of electrostatic interactions as the major stabilizing factor.