Carrel1 and Guthrie (’05) first suggested the possibility of transplanting a heart into man or the experimental animal. The advances in modern surgery have permitted the technical accomplishment of this objective in certain experimental animals. Yet, as with other tissues and organs, the “homograft reaction” remains a barrier to the complete realization of a functionally successful heart transplant (Bing et al.,’62).

Most of our present knowledge regarding the immunogenetic aspects of tissue transplantation has been accumulated from experimental studies utilizing tumors and skin grafts (Billingham and Silvers,’61); whereas the literature is relatively sparse concerning the transplantation characteristics of cardiac tissue. Several investigators have studied the transplantation of homografted hearts in dogs (Hume,’59). Poor (’57) grafted homologous fetal hearts into the cheek pouch of adult golden hamsters. Katzberg (’59) transplanted embryonic hearts into older chick embryos of the same strain; and Weinstein (’60) has studied the behavior of homografted hearts in the platyfish. Browning (’49) and Conway et al.(’58) have described the survival of homologous and isologous fetal and neonate heart transplants in the mouse. The former investigator studied the growth of intraocular transplants, whereas the latter determined the viability by the pulsatile activity of subcutaneous grafts observed through the Saran window modification of the transparent chamber technique. It was the purpose of this investigation to study the morphological and functional responses of cardiac tissue when transplanted to the ear of the mouse; and to correlate donor and host factors, as well