Background Hydrocyclones (HCs) have been recently extensively evaluated for their application in the separation of mammalian cells in perfusion bioreactors. The high centrifugal force derived within hydrocyclones is the key mechanism for cell separation inside such small-sized simple devices. This is usually accompanied by a very short residence time of the cells inside the separating equipment, usually not more than 0.2 s. Moreover, they have other specific characteristics, which highly recommend their application for the production of pharmaceutical products in perfusion cultivation bioreactors. They are characterized by their high performance, robustness, lack of movable parts, ease of in situ sterilization and suitability for cleaning-in-place processes.