Abstract: An apparatus and method for culturing cells includes a waste removal and product concentrating loop in which waste is removed from the medium and product produced by the cells is concentrated for harvesting. The apparatus includes a reservoir for retaining a supply of medium. The reservoir is in fluid communication with a cell culturing loop wherein medium flows from the reservoir to the cells and back to the reservoir. The medium waste removal and concentrating loop removes waste components produced by the cells and is in fluid communication with the reservoir such that medium is transferred from the reservoir to the waste removal and concentrating loop and back to the reservoir. The waste removal and concentrating loop includes a device having a semipermeable membrane. The medium flows along one side of the membrane for selective transfer of the waste components through the membrane.