A novel bioreactor system developed for high-density cultures of suspended mammalian cells is described using a tangential-flow filtration device outside the culture vessel to separate viable cells from spent medium. The filtration device is based on thin porous microfiltration membranes with a pore size of 0.20–0.65 μm. Because cells have a diameter of about 10–20 μm, they cannot permeate these membranes with the spent medium. So, allowing a perfusion culture to be created using this system. In most membrane filtration systems, clogging of the membranes has made long-term operation difficult. In this system, however, high pressure is not applied directly to the membrane, thus minimizing clogging. Also, clogging of the membrane was prevented by washing the membrane surface once a day, and increasing the membrane surface are. With this system, FM-3A cells were cultured and maintained at a high density of 3.0×107 cells/ml for two weeks, and a continuous culture was supported for as long as 34 days.