Polyions were tested for effects on some membrane-related functions. Both polycations investigated reduced the negative surface charge of assay cells and enhanced in vitro infectivity of murine C-type viruses, but had no influence on leukemia-virus-induced XC cell syncytia formation. Three polyanions increased the net outer cell charge, while only one of four inhibited infectivity and two of three impeded syncytia formation. Polyions had a slight, probably toxic, effect on the transmembrane potential, independent of their charge. Cells treated with fluorescent DEAE-dextran showed diffuse staining, which 4 h later had been modified into a granular fluorescence with unstained areas now present. This change correlated with a loss of enhancement of viral infectivity. The only polyanion which inhibited viral infectivity had a strong antihyaluronidase activity, and hyaluronidase and Ca++ both increased viral infectivity. It is suggested, therefore, that polyions may in part work on virus-cell membrane interactions by influencing membrane enzymes and not necessarily by simply changing the net outer cell surface charge.