According to the humoral theory of transplantation, antibodies cause allograft rejection. Publications are cited showing that antibodies: (1). cause hyperacute kidney rejection, (2). lead to C4d deposits associated with early kidney graft failures, (3). are a good indicator of presensitization leading to early acute rejections, (4). were present in 96% of 826 patients who rejected a kidney graft, (5). are associated with chronic rejection in 33 studies of kidney, heart, lung and liver grafts, and (6). in three studies, appeared in the circulation BEFORE evidence of bronchiolitis obliterans in lung transplants, and BEFORE kidney rejection. In addition, a prospective cooperative study of 1629 patients in 24 centers demonstrated that antibodies foretold subsequent failures after a follow-up period of 6 months (p = 0.05). The specificity of antibodies detected in the serum of rejecting patients were often not donor specific, presumably because they were absorbed by the rejecting organ. If the humoral theory is accepted, even provisionally, transplanted patients who have antibodies could be treated with immunosuppression until the antibodies disappear to determine whether chronic rejection can be blocked. If successful, in patients who do not have antibodies, immunosuppression could be reduced until antibodies appear.