In the present work, polyethersulfone ultrafiltration membranes were UV-initiated grafted with poly(ethylene glycol) methacrylate by varying the UV irradiation dose. The flux and selectivity performance of the prepared low-fouling thin-layer hydrogel composite membranes were tested in cross-flow filtration experiments and compared to unmodified commercial membranes with similar properties (water permeability and molecular weight cut-off). Here, the effects of feed composition (single proteins and protein mixtures), pH and cross-flow velocity were evaluated. Furthermore, the stability of these membranes was tested during three cycles of ultrafiltration and cleaning. The results showed that the performed surface hydrophilization increased the permeate flux and stabilized the membrane selectivity (relevant for separation of protein mixtures). The proteins charge was still important for flux and rejection during ultrafiltration with functionalized membranes. Increasing cross-flow velocity was more efficient for these membranes, since fouling effects were minimized by the grafted hydrogel layer. Furthermore, the membrane stability after three cycles of ultrafiltration and chemical cleaning at pH = 13 was proven and the cleanability of the hydrogel composite ultrafiltration membranes was much better compared to that of comparable unmodified polyethersulfone ultrafiltration membranes. (C) 2012 Elsevier B.V. All rights reserved.