A type of bioreactor in which hollow fibre membranes are used to transport fresh nutrients or oxygen to the medium, or to remove products or waste, or to immobilize biocatalysts such as enzymes and mammalian cells. The hollow fibres are porous semi-permeable membranes and packed into bundles within a cylindrical container. The pore size is sufficiently small to retain cells but sufficiently large to permit the passage of nutrients and waste materials. The hollow fibre bioreactor can be operated continuously. Examples include the cultivation of mammalian cells for the production of monoclonal antibodies, the growth of liver cells (hepatocytes), and for small-scale industrial waste treatment.