Hollow-fiber mammalian cell culture systems were first conceived (1) to mimic the in vivo cell environment. In tissues, cells exist immobilized at high density, and are perfused via capillaries having semipermeable walls. Fluid (blood) circulating within the capillaries brings oxygen and nutrients and removes CO₂ and other waste products. This description applies equally to hollow-fiber culture systems but with culture medium in the place of blood, and with capillaries made from ultrafiltration or microfiltration membranes.