PVDF microfiltration membranes were modified by photografting of 4-vinylpyridine and then quaternized with n-butyl chloride. Lysozyme was employed as a model alkaline protein to investigate the antifouling performance of quaternized membranes. The effects of grafting and quaternization conditions on modification were studied. The results show that, the percent of initiator grafted and grafting density increase with the irradiation time, whereas the grafted chain length and grafting degree rise initially and then reach plateau. The optimal concentrations of initiator and 4-vinylpyridine are 5 wt.% and 20 wt.%, respectively. The ion exchange capacity increases with quaternization temperature and time, and the optimal concentration of n-butyl chloride is 30 wt.%. When the solution pH is at the isoelectric point of lysozyme, the saturated adsorption capacity of membranes reaches maximum. When the solution pH is lower than isoelectric point, the saturated adsorption capacity of quaternized membranes is the lowest due to electrostatic repulsion and hydrophilicity of membranes. The modified membranes exhibit excellent antifouling performance to alkaline proteins. (C) 2013 Elsevier B.V. All rights reserved.