Cartridge filters are used in many industries to prefilter, clarify or decontaminate a wide variety of liquids. Among many others, the filtration efficiency and retention capacity are the two most important characteristics of a cartridge filter. Several test procedures are available to evaluate and compare these characteristics. However, the standardised tests are generally not applicable, because they are obsolete or not suited to water and chemicals filtration; and other tests are used by individual filter companies, and are not agreed by competitors or by users. The Institut de la Filtration et des Techniques Separatives (IFTS) has developed a new test method. This allows the measurement of efficiency by on-line counting of particles down to 1 mum, both upstream and downstream of the filter, during the test period, until clogging (which occurs in a rather short time, less than 6 hours). This has been made possible by challenging the filter with periodic fluctuations of the upstream concentration (alternately high and low). Particle counting is processed continuously downstream and upstream only during the periods of low concentration. Many results have been obtained on different kinds of cartridge filters, and have proved the sensibility, reproducibility and reliability of the new method.