Immunological considerations of special interest in liver transplantation are examined. It is concluded that liver transplants between human beings generate typical rejection reactions although they may, overall, be somewhat more mild than those encountered in transplanted kidneys and hearts. This is the case in some animals for reasons which are not yet clear. There is evidence that the transplanted liver can suffer immune attack better than other organs. Whether graft-vs.-host activity is of clinical importance is uncertain. The advent of cyclosporin immunosuppression has improved the outlook for liver recipients but multiple complications of immunosuppression will still occur and demand skillful management.