Expression of viral fusogenic membrane glycoproteins (FMGs) is a potent strategy for antitumor cytotoxic gene therapy in which tumor cells are fused into large multinucleated syncytia. To understand how local cell killing can potentiate activation of antitumor immune responses, we characterized the mechanism of FMG-mediated cell killing. Here, we show that syncytia are highly ordered structures over 24-48 h but then die through processes that, by multiple morphological and biochemical criteria, bear very little resemblance to classical apoptosis. Death of syncytia is associated with nuclear fusion and premature chromosome condensation as well as severe ATP depletion and autophagic degeneration, accompanied by release of vesicles reminiscent of exosomes (syncytiosomes). Dying syncytia produce significantly more syncytiosomes than normal cells or cells killed by irradiation, freeze thaw, or osmotic shock. These syncytiosomes also load dendritic cells (DCs) more effectively than exosomes from cells dying by other mechanisms. Finally, we demonstrate that syncytiosomes from either autologous or allogeneic fusing melanoma cells lead to cross-presentation of a defined tumor antigen, gp100, by DCs to a gp100-specific CTL clone. Cross-presentation was significantly more efficient than that with exosomes from normal, irradiated, or herpes simplex virus thymidine kinase/ganciclovir-killed tumor cells. Therefore, FMG-mediated cell killing combines very effective local tumor cell killing with the potential to be a highly immunogenic method of cytotoxic gene therapy. In addition, these data open the way for novel methods of loading DCs with relevant tumor-associated antigens for vaccine development.