SUMMARYIn a series of 67 orthotopic hepatic allografts in the baboon, 75% of the untreated animals died in the first 2 weeks after transplantation. One untreated animal survived for 36 days. Continued administration of horse antibaboon lymphocyte globulin (ALG) prolonged allograft survival, but this effect was somewhat inferior to that resulting from azathioprine and prednisolone administration, and the addition of ALG to these immunosuppressive drugs did not produce an appreciable additive effect. The administration of semisoluble donor liver extract after transplantation in combination with ALG or ALG and azathioprine did not result in survival data better than those obtained with the immunosuppressive agents alone. Administration of ALG for only 6 days after transplantation, followed by a single injection of 25 X 106donor bone marrow cells and no further immunosuppression, was superior to the continued use of ALG and was equal to or slightly better than continued azathioprine and prednisolone treatment. Preliminary studies with serial mixed leukocyte cultures and mixed cell agglutination with suspensions of rejecting hepatocytes may indicate a state of “:partial tolerance”: and/or enhancement in animals receiving a limited course of postoperative ALG administration followed by 25 X 106donor bone marrow cells.