LEUKOCYTES are known to carry transplantation antigens both in animals1 and in man.2,3 Should transfusions of blood from one individual to another, therefore, be considered as a homograft and to what extent do blood transfusions sensitize for a subsequent graft? It is known that whereas in stored blood the leukocytes are not well preserved,4-6 they survive nicely in freshly drawn heparinized blood.7,8 It is not surprising, therefore, that leukocyte antibodies have been demonstrated following massive transfusions of fresh blood containing living leukocytes. Using the approach of Walford and his colleagues,8-10 we have sought to further investigate this problem by examining the sera of patients undergoing open heart surgery. During cardiopulmonary bypass, the patient receives a large quantity of fresh whole blood. Such a transfusion given to an immunologically competent individual should constitute, therefore, a homograft stimulus of some magnitude. With this in mind, we have