The fractionation of two proteins, either positively charged (Lysozyme) or negatively charged (bovine serum albumin, BSA) was investigated by varying the ionic strength with unmodified or positively charged inorganic ultrafiltration membranes. Chemical modification was obtained by coating of polyvinylimidazole which amine groups reacted further with bisepoxiranes in order to have both partly quaternized amine group and pH stable network on membrane surface. The retention of the single protein decreases, with ionic strength when electrostatic interactions between the free protein in the bulk and the adsorbed protein onto the membrane are occurring. An increase of single protein retention with modified membranes appears at high ionic strength due to hydrophobic interactions between proteins and polymer coating. For protein mixtures, at low ionic strength (0.015), observed selectivities are more than 10 whatever the membrane used; both membrane fouling and protein-protein interactions occur in the protein mixture. At intermediate (0.25 M) and high (1 M) ionic strengths, the observed selectivity with modified membranes remains stable (about 6) whereas selectively with unmodified membranes decreases and is close to size selectivity. Hence the fractionation of proteins with modified membranes remains satisfactory in the entire range of ionic strengths due to ionic and salt promoted interactions between protein and membranes.