An upflow packed-bed cell recycle bioreactor (IUPCRB) is proposed for obtaining a high cell density. The system is comprised of a stirred tank bioreactor in which cells are retained partially by a packed-bed. A 1.3 cm (ID) X 48 cm long packed-bed was installed inside a 2 L bioreactor (working volume 1 L). Continuous ethanol fermentation was carried out using a 100 g/L glucose solution containing Saccharomyces cerevisiae (ATCC 24858). Cell retention characteristics were investigated by varying the void fraction (VF) of the packed bed by packing it with particles of 0.8 similar to 2.0 mm sized stone, cut hollow fiber pieces, ceramic, and activated carbon particles. The best results were obtained using an activated carbon bed with a VF of 30 similar to 35%. The IUPCRB yielded a maximum cell density of 87 g/L, an ethanol concentration of 42 g/L, and a productivity of 21 g/L/h when a 0.5 h(-1) dilution rate was used. A natural bleeding of cells from the filter bed occurred intermittently. This cell loss consisted of an average of 5% of the cell concentration in the bioreactor when a high cell concentration (approximately 80 g/L) was being maintained. (C) KSBB.