This study was designed to determine optimal operating conditions of a bioartificial liver (BAL) based on mass transfer of representative hepatotoxins and mediators of immune damage. A microprocessor controlled BAL was used to study mass transfer between patient and cell compartments separated by a hollow fiber membrane. Membrane prermeability (70, 150, or 400 kDa molecular weight cut-off-MWCO), membrane convection (high: 50 mL/min; medium: 25 mL/min; low; 10 mL/min; diffusion: 0 mL/min); and albumin concentration in the cell compartment (0.5 or 5 g%) were considered for a total of 24 test conditions. Initially, the patient compartment contained pig plasma supplemented with ammonia (0.017 kDa), unconjugated bilirubin (0.585 kDa), conjugated bilirubin (0.760 kDa), and pig IgM (900 kDa). Mass transfer of each substance was determined by its rate of appearance in the cell compartment. Membrane fouling was assessed by dextran polymer technique. Of the three tested variables (membrane pore size, convection and albumin concentration), membrane permeability had the greatest impact on mass transfer (P < 0.0001). Mass transfer of all toxins was greatest under high convection with a 400kDa membrane. Transfer of IgG and IgM was insignificant under all conditions. Bilirubin transfer was increased under high albumin conditions (P = 0.055). Fouling of membranes ranged from 7% (400 kDa), 24% (150 kDa) to 62% (70 Da) during a 2-h test interval. In conclusion, optimal toxin removal was achieved under high convection with a 400-kDa membrane, a condition which should provide adequatie immunoprotection of hepatocytes in the BAL. Biotechnol. Bioeng. 2009;104: 995-1003. (C) 2009 Wiley Periodicals, Inc.