SUMMARYProlongation of survival time of cardiac allografts in rats by the previous i.v. injection of donor strain blood is described. Modest prolongation of survival time of split skin grafts is also demonstrated. A similar effect on cardiac allografts occurs when donor strain thoracic duct lymphocytes are employed instead of blood. Donor strain plasma and red cells are shown to be ineffective. Passive transfer of serum from recipient strain animals “conditioned” with donor strain blood does not produce prolongation of survival time of cardiac allografts. Perfusion of the coronary arteries with such serum before transplantation also has no effect. Rats with long surviving cardiac allografts are shown to reject donor strain split skin grafts in a manner similar to that of control rats.