The aim of the present study was to assess the survival of adult porcine islets transplanted into baboons receiving either (1) conventional triple drug immunosuppressive therapy or (2) a non-myeloablative regimen and an anti-CD154 monoclonal antibody (mAb) aimed at tolerance-induction. Group 1 baboons (n = 3) were pancreatectomized prior to intraportal injection of 10 000 porcine islet equivalents (IE)/kg and immunosuppressed with anti-thymocyte globulin (ATG), cyclosporine and azathioprine. In Group 2 (n = 2), non-pancreatectomized baboons underwent induction therapy with whole body and thymic irradiation, and ATG. Extracorporeal immunoadsorption (EIA) of anti-Galα1,3Gal (Gal) antibody was carried out. Maintenance therapy was with cobra venom factor, cyclosporine, mycophenolate mofetil, methylprednisolone and anti-CD154 mAb. Porcine islets were injected intraportally (14 000 and 32 000 IE/kg, respectively) and high-dose pig mobilized peripheral blood progenitor cells (3 × 1010 cells/kg) were infused into a systemic vein. Porcine islets were also implanted in the sternomastoid muscle to facilitate subsequent biopsies. In both groups, porcine C-peptide was measured, and histological examination of liver or sternomastoid muscle biopsies was performed at regular intervals. In Group 1, total pancreatectomy reduced human C-peptide to < 0.1 ng/ml and induced insulin-requiring diabetes. The transplantation of porcine islets was followed by normalization of glycemia for 15-24 h. Porcine C-peptide was detected only transiently immediately after porcine islet injection (maximum 0.12 ng/ml). Histological examination of liver biopsies taken between days 2 and 19 did not reveal viable islets, but necrotic cell structures with mononuclear cell infiltrates were identified in portal venules. In Group 2, injection of porcine islets into non-pancreatectomized recipients induced a transient hypoglycemia (2-4 h) requiring concentrated intravenous dextrose administration. Porcine C-peptide was detectable for 5 and 3 days (maximum 2.8 and 1.0 ng/ml), respectively. Baboon #4 died on day 12 from small bowel intussusception. Liver and sternomastoid muscle biopsies showed well-preserved porcine islets, staining positive for insulin and glucacon, without signs of rejection. In baboon #5, viable islets were detected in the sternomastoid muscle biopsy on day 14, but not on day 28 or thereafter. A progressive mononuclear cell and macrophage infiltration was seen in the biopsies. In conclusion, conventional immunosuppression allowed survival of porcine islets in baboons for > 24 h. The non-myeloablative regimen prolonged survival of porcine islets for > 14 days. However, despite depletion of T cells, anti-Gal antibody and complement, and CD154-blockade, porcine islets were rejected by day 28. These results suggest that powerful innate immune responses are involved in rejection of discordant xenogeneic islets.