SUMMARY Cardiac allografts were studied with a model previously established for passive enhancement of renal transplants in the inbred rat. Like kidney grafts, they survived for prolonged periods in recipients of antiserum from syngeneic animals sensitized against donor antigen. The survival of skin grafts was prolonged only slightly in hosts bearing enhanced organs. The active production of alloantibody by recipients of long surviving heart grafts was demonstrated. This antiserum, produced by such animals against a heart or kidney graft, was not organ-specific and provided effective enhancement when transferred into recipients of the opposite organ. Allografts placed in presensitized animals were rejected in an accelerated manner. Those challenged with test skin grafts were relatively unaffected, although several well functioning grafts experienced serious but self-limiting rejection episodes. Splenectomy at the time of transplantation, or performed 75 days postoperatively, did not affect the overall survival of enhanced heart grafts, although several of these organs underwent spontaneously reversing rejections. The immunological competency of spleen cells from enhanced recipients of cardiac grafts was found unaffected in a popliteal lymph node graft-versus-host assay. With this transplantation model, several parallels to the phenomenon of enhancement in tumour systems were drawn.