Current integrity tests are not sufficiently sensitive to detect nanometric scale breaches in the active layer of ultrafiltration membranes. The current paper describes a new approach for the detection of such breaches. We introduce two representative types of nanoprobes emulating virus transport over the membrane. Gold nanoparticles and fluorescent-dye-labeled MS2 bacteriophages were introduced for seeding tests. The affordability of the former probe has become more realistic with the development of electrochemical detection of gold nanoparticles by anodic stripping voltammetry (ASV). Our ASV system showed high sensitivity, of the order of single parts per billion, indicating the feasibility of developing the experimental protocol for on-site analysis. However, the physical characteristics of gold nanoparticles and particularly their specific density differ from those of viruses. The fluorescent bacteriophage probe emulates viral transport much better, though this technique is less sensitive, and further lowering of the limit of detection is required. Application of the probes for testing membrane integrity will provide a basis for developing on-line testing for removal of virus-sized particles and in addition to being a valuable research tool may provide a means to confirm compliance of membrane systems with the stringent regulatory requirements of the drinking water industry. (c) 2005 Elsevier B.V. All rights reserved.