Background. Exposing adult porcine pancreatic islets (PI) to xenoreactive natural antibodies (XNA) induces brisk inflammatory injury that involves activation of the complement system. Gene transfer of Bcl-2 has been shown to protect PI from apoptosis and necrosis in several models. In this study, we investigated the effect of Bcl-2 gene transfer on protection of PI from primate XNA and complement-mediated injury. Methods. The PI were isolated from adult female sows. Only islet preparations that exhibited >90% viability and purity were used. Fresh rhesus monkey serum served as the XNA source. Gene transfer of Bcl-2 was achieved with an adenoviral vector (AdBcl-2) at 500 particle forming units (pfu)/cell. The Bcl-2 expression was confirmed by Western blot technique. Untransfected and transfected PI were incubated in 50% fresh complete serum (CS) or heat-inactivated (HI) rhesus serum for 24 hours. The PI viability was analyzed with acridine orange and ethidium bromide staining. Antibody and complement-mediated cytotoxicity were tested by intracellular lactate dehydrogenase (LDH) release. The PI function was assessed in vitro by static incubation studies and in vivo after intraportal transplantation in diabetic severe combined immunodeficiency (SCID) mice. Results. The AdBcl-2 gene transfer resulted in Bcl-2 gene expression in >90% of PI cells. Following exposure to XNA, <15% of the untransfected cells were viable. Similar results were obtained in PI transfected with a similar recombinant adenovirus encoding the reporter gene E coli β-galactosidase (AdLacZ), an irrelevant gene. A significant increase in LDH release was observed in control PI after exposure to CS compared with PI that overexpressed Bcl-2 (82.89% ± 7.78% vs 34.31% ± 5.4%, P < .005). Higher insulin release was observed in vitro in PI transfected with Bcl-2 compared with untransfected PI or islets transfected with AdLacZ (stimulation index of 0.9 ± 0.31, 0.9 ± 0.3 vs 2.67 ± 0.4, respectively). Only PI treated with AdBcl-2 were able to achieve euglycemia after exposure to XNA and complement after transplantation. Conclusions. Transfer of the antiapoptotic and antinecrotic Bcl-2 gene into PI can reduce primate XNA and complement-mediated lysis. Cytoprotection of PI with Bcl-2 has potential to improve survival of PI xenotransplants.