A spinning cylindrical filter, known as a spinfilter, permits the mammalian cell bioreactor operation at high perfusion rates leading to very high cell densities (10(7) mL-1). Filter screens with openings (25 mum) slightly larger than the average cell size have been used to retain single cells in suspension over a long period of operation without clogging. We have previously shown why it is necessary to optimize the rotational speed of the spinfilter in order to achieve efficient cell retention and avoid potential screen clogging. Effects of bulk mixing and perfusion rate on screen fouling and cell retention were also investigated. Based on this analysis, in this article, we suggest strategies for scaleup of spinfilters. Experimental data from 12- and 175-L (working volume) bioreactors is shown in support of the scaleup analysis.