A U.S. Patent Application for “Perfusion Filtration Systems” was published by Artemis Biosystems on October 3rd, 2019. The present invention is a method of filtering a cell culture media comprising passing a cell culture media comprising mammalian cells at a viable cell density of at least 10.times.10.sup.6 cells per mL and a virus through a plurality of tubular porous membranes of a filter elements enclosed within a casing, wherein each tubular porous membrane filter element comprises a plurality of tubular porous membranes, wherein at least 95% of the pores in each of the plurality of tubular porous membranes are between 5 .mu.m and 15 .mu.m in size. Learn More