An alternative ultrafiltration membrane integrity test is presented and evaluated, based on the use of magnetic nanoparticles (Fe3O4) and measurement of magnetic susceptibility. The mean size of nanoparticles used is around 35 nm and they show a good disparity between 20 and 100 nm. A series of integrity tests were carried out on a Norit membrane module containing 100 fibers under a low transmembrane pressure of 0.25 bar. The results showed that no magnetic susceptibility was detected in permeate when the tests were performed on the intact module in both cross-flow and dead-end filtration, indicating the complete nanoparticle retention by the intact module. However, when even one fiber was broken in the module (1% breakage rate), magnetic susceptibility of permeate could be detected instantaneously even at feed concentrations as low as 1.2 ppm with Bartington magnetic susceptibility meter. This detection is valid during all the filtration process. The results also showed that the membrane permeability could be completely recovered after a backwash. This membrane integrity test, with the advantages of simplicity, on-line operation, high detection specificity and sensitivity, quick detection and very low influence on membrane fouling, seems to be suitable for large-scale drinking water plants.