One billion cells or more can be cultured over long periods in a 20 ml FiberCell hollow fibre bioreactor. This customer review describes how the system is used for harvesting large amounts of recombinant protein. The same principles apply for harvesting any factors secreted by mammalian cells. N.B. the 70 ml FiberCell cartridges enable up to 1×10^11 cells to be cultured at ultra-high density.