Abstract: The invention presents a method for the culturing of mammalian cells. This method involves the use of a bioreactor, which contains a sample of mammalian cells in a culture medium containing large molecules. Positioned inside the bioreactor is a semipermeable membrane which defines a space separated from the bioreactor by the semipermeable membrane. A nutrient medium flows through this separated space and, via virtue of the semipermeable nature of the separating membrane, nutrient pass therethrough into the culture medium, while cellular waste products pass into the separated space. The semipermeable membrane is selected so that the cells and large molecules, such as proteinaceous materials, cannot pass through the membrane, but remain in the bioreactor.