To enable high density culture of hepatocytes for use as a hybrid artificial liver support system or a bioreactor system, a packed-bed reactor using collagen-coated reticulated polyvinyl formal (PVF) resin was applied to a primary culture of hepatocytes. Cubic PVF resins (2 x 2 x 2 mm, mean pore size: 100, 250 or 500 microns) were used as supporting substrates to immobilize hepatocytes. Two hundred and fifty cubes were packed in a cylindrical column, and 2.6-11.3 x 10(7) hepatocytes were seeded in the column by irrigating with 3 ml of the medium containing hepatocytes. Perfusion culture experiments using this packed-bed reactor, as well as monolayer cultures using conventional collagen-coated petri dishes as control experiments, were performed. Sufficient amounts of hepatocytes were found to be immobilized in the reticulated structure of the PVF resins. The highest density of immobilized hepatocytes attained with PVF resin was 1.2 x 10(7) cells/cm3 PVF, which showed levels of ammonium removal and urea-N secretion comparable to those in the monolayer culture. It is concluded that the packed-bed reactor system utilizing PVF resin is a promising process for developing a bioreactor or a bioartificial organ using hepatocytes.