Recent historic events in the surgery of heart transplantation have awakened the interest of allied specialties having a role in the postoperative care of the recipient patient. Radiology has an important share in this responsibility and the radiologist is an important team member in the transplantation effort. The importance of diagnostic radiologic studies of the heart needs no comment. Little is known, however, of the importance of radiology in the postoperative course of the transplanted heart, particularly in respect to the recognition of rejection phenomena. At the present time the limited experience of the radiologist in heart transplantation prohibits radiologic conclusions regarding the acceptance or rejection of a heart. The immunologic responses of the body to foreign tissue are complicated and poorly understood, and at the present time the surgical technic of heart transplantation has surpassed the immunological knowledge of host acceptance or rejection of the transplanted organ. Our own personal ignorance of immunology and of the complexities of tissue histocompatibility has, within the past few months, become pitifully obvious because of the heart transplantations carried out in our institution. Suddenly, in less than a four-month period 10 cardiac transplants have been performed at St. Luke's Episcopal Hospital in Houston, Texas, and the radiology department has been caught up in the wave of excitement and enthusiasm which accompanied each of these procedures. Five of these patients are presently living comfortably with their new hearts. The daily radiographic studies performed on these recipients have enabled us to recognize the importance of radiology in the postoperative course and to correlate the radiographic features with the clinical responses. This experience has also demonstrated the necessity of the radiologist having at least a conversational understanding of the antigen-antibody complexities entering into the donor-recipient relationship. Although the immunology of tissue rejection has been appreciated for many years, it was not until the early 1940's that Medawar conclusively showed by experimental evidence that tissue rejection followed immunologic laws and that the transfer of immunity was by cells and not by humoral mechanisms as previously thought. In any immunologic reaction, antigens reach the immunological responsive center of the body (the reticuloendothelial system) by a poorly understood afferent limb which apparently involves small lymphocytes, macrophages, and perhaps other cells. After the responsive centers have been sensitized, antibodies are carried back to the graft site by an efferent pathway consisting of small lymphocytes and histiocytes, thereby producing the rejection reaction.