The use of transgenic cells transplanted in syngeneic rodents has shown modest success, but allogeneic and xenogeneic transplants have not been uniformly successful. To assess the feasibility of xenogeneic and allogeneic myoblast transplantation, we subjected seven adult swine to transplantation of murine atrial tumor cells (xenogeneic), neonatal porcine myocytes (allogeneic), and human fetal cardiomyocytes into the left ventricular wall. After general anesthesia, isolated cells were injected along the anterior and posterior walls of the porcine left ventricle. All the animals were immuno-suppressed and observed for 1 month after injection, at which time they were killed and analyzed. This report will present results primarily concerned with the success of human cell transfers. In all injected sites examined, the transplanted cells thrived within the host myocardium with no significant rejection. Transplant cells formed close associations with host myocytes that resembled nascent intercalated disks on electron microscopy. These cells also contained myofibrils and other cell architecture resembling the transplanted cell lines. Additionally, these cells appeared to produce an angiogenic influence resulting in the proliferation of the surrounding microvasculature. We believe that these findings indicate successful xenogeneic and allogeneic myoblast cell transplantation in a large animal model. These experiments set the stage for future studies to assess the ability of these cells to form a syncytium, contract, and potentially repair failed myocardium.