A new, hollow fiber bioartificial liver (BAL) was tested in an anhepatic rabbit model to assess the hollow fiber membrane as an immunologic barrier. The extracorporeal BAL contained rat hepatocytes (xenocytes) entrapped in collagen gel inside the lumen of hollow fibers with 100 kd nominal molecular weight cut-off. Blood from the anhepatic rabbit flowed in the extracapillary compartment. After 4 h of BAL hemoperfusion, the hepatocyte gels were tested for evidence of rabbit immunoglobulin and complement. Samples of the gels were stained with fluorescein isothiocyanate (FITC)-labeled antibodies directed against rabbit IgM, rabbit IgG (Fc and F[ab]2), and rabbit complement (C3) and studied by immunofluorescence microscopy. Enzyme-linked immunosorbent assay and Western blotting were used, respectively, to quantify and identify proteins inside the BAL and in the rabbit blood. These studies suggest that very little rabbit IgG (primarily fragments of IgG) and no IgM or C3 crossed the hollow fibers during hemoperfusion. Rat albumin steadily accumulated in the rabbit blood during hemoperfusion, which indicated membrane permeability to molecules of that size and stable hepatocyte function by the BAL. The authors conclude that hollow fibers with 100 kd nominal molecular weight cut-off provide immunoprotection for xenocytes in an extracorporeal BAL.