Antibodies have the potential to be immunotherapeutic agents, used either as stand-alone therapy or as an adjunct for managing chronic viral infection. In addition, antibodies may be used prophylactically in individuals who have been accidentally exposed to hepatitis B virus (HBV) or hepatitis C virus (HCV), or to prevent re-infection of the liver in patients who have undergone liver transplantation. Human monoclonal antibodies to HBV and HCV were generated and their ability to reduce viral load was tested in different animal model systems, the Trimera mouse model and HBV-carrier chimpanzees. These antibodies were further developed and are currently being studied in clinical trials for chronic HBV or HCV and in liver transplant patients. The antibodies were shown to be safe, tolerable and could significantly reduce viral load. Their ability to inhibit HCV re-infection in the transplanted liver is being evaluated.