The use of cells from xenogeneic origin in a bioartificial liver can have a number of immunological consequences, not only for the cells in the bioartificial liver but also for the patient receiving the bioartificial liver treatment. The impact of these consequences will depend on the immune status of the patient receiving bioartificial liver treatment, the duration and frequency of the treatment and on the extent of interaction between the patients blood (or plasma) and the xenogeneic liver cells. In an experimental model we infused rats with a culture supernatant of pig hepatocytes and demonstrated using Western blots and immunohistological techniques that antibodies are raised against the very small amounts of the pig hepatocyte-derived proteins present in the culture medium. Potential problems of bioartificial liver destruction and the possibility of hypersensitivity reactions due to the secretion of xenogeneic proteins into the circulation of the patient are discussed. Because the liver has an important role in the clearance of immune complexes it is concluded that precautions should be taken when (repeated) application of a xenogeneic bioartificial liver in patients with liver failure is considered.