It is not likely that in the near future we will be able to transplant tissues from one unrelated individual, or one species to another and have them survive indefinitely. There are still great gaps in our knowledge regarding the relationship and make-up of cells and organs that prevents a completely successful homoiotransplantation of tissues. Hundreds of workers using all types of vertebrate and invertebrate animals and tissues are, however, painfully and slowly adding building stones of information to our foundation of knowledge. We are assembling data about the individuality of tissues. The organ used in this investigation was the heart of a pup. The heart was transplanted by vascular anastomosis into the neck of an adult dog. The heart was used because its viability could be readily estimated after transplantation, by noting the character of the pulsations. The dog also, according to the work of Olson (1940), has isohemagglutinins present in blood plasma but not in the serum and due to the weakness of the natural isohemagglutinins, a transfusion of incompatible blood will produce no symptoms so the transplantation of the hearts could be performed in the dog and the typing of blood dispensed with. Other workers have transplanted mammalian hearts but their work was performed before the discovery of antibiotics. In this investigation I hoped to show that the use of penicillin preoperatively to the adult and pup, and post-operatively to the adult, would increase the active lifespan of transplanted hearts. Other workers have attempted to modify the differentials between a transplanted tissue and its host. Some of the methods used included Roentgen rays, graded exposure to heat, immunization with homoiogenous tissue and freezing. Since the body fluids appear to influence the rate of deterioration of a transplant, it was thought that transfusions from the adult to the donor pup would perhaps attenuate the differentials. In a number of animals transfusions were performed. It was hoped that improvements could be made in the surgical technique and that by the use of penicillin and transfusions a mammalian heart could be kept active and beating for a longer period of time than previously.