The aim of this study was to evaluate how defects in a membrane surface can affect the molecular weight cut-off (MWCO) and the membrane permeability as they are traditionally defined. These two parameters are important criteria for the selection of ultrafiltration membranes with regard to efficiency and rejection of dissolved and particulate substances and especially disinfective retention. Tests were conducted with a flat-sheet membrane with an effective filtration area of 1.45 x 10(-3)m(-2). Dead-end filtration experiments were performed with various feed solutions: pure water to measure the hydraulic permeability, a mixture of dextrans to determine the apparent MWCO and protein solutions. This was done before and after the membrane integrity was altered by perforating its surface with a sharp tip.