Surface charge characteristics of filters capable of adsorbing viruses were studied. This was accomplished by microelectrophoresis of filter particle suspensions. The effect of pH on the surface charge properties of (1) cellulose nitrate, (2) cellulose acetate, and (3) fibreglass/epoxy filter materials was examined. It was discovered that these filter materials exhibited net negative charges over the pH range 2-7 when suspended in KC1/HC1 (or NaOH) solutions of ionic strength .2, with net negative charge approaching neutrality at the lower pH levels. The effects of various salts (KC1, MgC, CaC, AlCl) on the charge properties of cellulose nitrate were examined at pH 3.5 and pH 7.. It was observed that at both pH levels, increasing ionic strength caused the net negative charge of the filter material to approach neutrality. At a given ionic strength, the divalent and trivalent cations were more effective at decreasing the net negative charge of cellulose nitrate than the monovalent cations. The aluminum cations were actually capable of reversing the net charge of the filter material, from negative to positive. It was concluded that trivalent cations, and possibly divalent cations, are capable of complexing with specific groups on the filter surface. A mechanism for adsorption of viruses to filter surfaces was proposed. Complex formation between multivalent cations and specific groups on both viral and filter surfaces reduces the zeta potentials. This, coupled with a possible ionic strength effect on the configuration of the double layers, facilitates the approach of the two surfaces, allowing van der Waals attractive forces to exert a significant effect. With multivalent cations there is a further distinct possibility of cross-complexation between the two surfaces. It is proposed that cross-complexation by multivalent cations is more important than van der Waals forces in promoting attachment of the two surfaces, and that the action of the cations might be described in terms of a "cationic adhesive" effect. Electrostatic forces would be expected to oppose, but not prevent, cation promoted adhesion at neutral pH, when both virus and filter exhibit net negative charges. However, at pH 3.5, when the virus surface becomes net positively charged, electrostatic attractive forces would add to the "cationic adhesive" effect, resulting in highly efficient adsorption of viruses to filter surfaces.