Heterotopic cardiac allografts were performed in unrelated Large White piglets to test (i) if such a technique could be used in young (12 days) and neonates (less than five days); (ii) if tolerance to histocompatibility antigens could be induced by donor-cells injection to piglets that were immunosuppressed with heterologous antilymphocyte serum (ALS). The experimentation included two different series. The first one involved ten allografts done in early weaned (10 days), 12-day-old piglets (mean live weight 3.5 kg). They received or not ALS 2 ml per kg per day by subcutaneous route during five consecutive days after transplantation. In addition, two of the ALS treated piglets received endovenously, on day 7 after transplantation, cryoconservated cells from the spleen of the heart-donor. The second series involved ten allografts performed in suckling piglets just after birth at a mean live weight of 1.4 kg. These animals were first thymectomized at day 0 (6 h to 44 h after birth), and then received ALS (4 ml per kg by intraperitoneal route) on day 1. The heterotopic cardiac allograft was performed on day 2. Out of them, two received endovenously, on day 3, conservated splenic cells from the heart donor. In both series the heterotopic cardiac allograft was done, after right nephrectomy, anastomosing the donor thoracic aorta end to side to the recipient abdominal aorta, and the donor pulmonary artery end to side to the recipient inferior vena cava. From the surgical point of view, it appeared that the heterotopic cardiac allograft was easy to perform, and successful in most cases, in 12-day-old piglets. The same operation in neonates was as easy to perform as in older animals, but most failed to survive due to hemorragic leakage and post operative shock (repetitive anesthesia, insufficient suckling). From the immunological point of view, we failed to obtain any active enhancement of heart allograft with significant prolongation of the survival of the allogenic heart allograft (electro-cardiogram from epicardic electrodes). Rejection usually occurred in 6 to 8 days whatever the treatment (ALS with or without splenic cells from the donor, or nothing).