Ultrafiltration of either single protein solutions (lysozyme 14,300 g mol(-1), pI = 11: lactoferrin 80,000 g mol(-1), pI = 8-9) or mixed protein solution was performed with inorganic membranes (MMCO 300.000 g mol(-1),pore radius 14 nm) chemically modified in order to bear either pyrophosphate (PP, anionic) or ethylenediamine (EDA, cationic) groups. The electrophoretic mobility of modified and unmodified zirconia particles fouled with proteins was similar whatever the grafted groups, meaning that the membrane surface was always made of adsorbed proteins during UF In spits of that, for the UF of lysozyme/lactoferrin mixed solution. the maximum selectivity (S = lysozyme transmission/lactoferrin transmission = 165) was observed with the EDA membrane and allowed an instantaneous purity of lysozyme in the permeate close to 100% to be achieved. Such high selectivity was mainly due to the negligible transmission of lactoferrin with the membrane modified with the EDA groups in the ionic strength range 0-100 mmol l(-1) of NaCl at pH 7 (achieved either for mixed and single solutions).