Although partial penetration of ultrafiltration membranes by viruses and bacteria is well documented, there is no satisfactory explanation for the phenomenon. The current study seeks a possible explanation by studying the retention of viruses at different transmembrane pressures (TMPs). In contrast to previous predictions, higher TMPs lead to reduced virus retention levels. Based on indirect evidences it is proposed that the penetration occurs because of the formation during operation, rather than the initial presence, of abnormally large pores. It is therefore suggested that since pore enlargement is induced by high TMPs, high virus retention levels can be obtained only at low TMP values. (c) 2008 Elsevier B.V. All rights reserved.