Pleating of membranes allows a larger surface area to be packed into small volume modules and is an integral feature of cartridges used in a range of biotechnology applications. This work investigates the effect that pleating has upon cartridge performance. Using specially fabricated 2.54 cm (1 '') sections of a large scale pleated membrane cartridge, together with a full scale 25.4 cm (10 '') cartridge, the pleat height (h(p)), pleat type, and pleat packing density (PPD), defined as the number of pleats packed around a central core, were systematically varied so as to investigate the subsequent effect upon filtration performance assessed in terms of clean water flux. The normalised clean water flux of a Supor (R) UEAV 0.2 mu m rated 10 '' pleated cartridge filter was 53% lower than that measured for a flat sheet disc of the same membrane material. By reducing both PPD and h(p), the measured clean water flux increased, such that a cartridge with a PPD of 0.65 and a h(p) of 10 mm achieved a comparable clean water flux to that of the flat sheet membrane. The measured variation in flux is attributed to changes in the ease by which the process fluid can access the inner surface of each membrane pleat. This was confirmed by using a dilute 7.7 g l(-1) yeast suspension as a probe to investigate the penetration of micron-sized (similar to 2 mu m) particles into the base of the individual membrane pleat. This showed that at PPD values beyond 0.85 and at an h(p) of 10 mm, areas of the cartridge were largely inaccessible to the yeast cell challenge due to the effects of pleat crowding. As the PPD and h(p) were reduced, accessibility of the particles to the inner surface of each pleat improved until the whole membrane surface became covered with yeast. Whilst packing more membrane into a cartridge via pleating will improve the overall performance for a given feedstock, these results show that an increase in membrane area may not lead to a proportional increase in membrane performance. The results may provide a basis for the development of models for the prediction of pleated cartridge membrane performance. (C) 2009 Elsevier B.V. All rights reserved.