The location of immune activation is controversial during acute allograft rejection and unknown in xenotransplantation. To determine where immune activation to a xenograft occurs, we examined whether splenectomized alymphoplastic mice that possess no secondary lymphoid organs can reject porcine skin xenografts. Our results show that these mice rejected their xenografts, in a T cell-dependent fashion, at the same tempo as wild-type recipients, demonstrating that xenograft rejection is not critically dependent on secondary lymphoid organs. Furthermore, we provide evidence that immune activation in the bone marrow did not take place during xenograft rejection. Importantly, immunity to xenoantigens was only induced after xenotransplantation and not by immunization with porcine spleen cells, as xenografted mutant mice developed an effector response, whereas mutant mice immunized by porcine spleen cells via i.p. injection failed to do so. Moreover, we provide evidence that antixenograft immunity occurred via direct and indirect Ag presentation, as recipient T cells could be stimulated by either donor spleen cells or recipient APCs. Thus, our data provide evidence that direct and indirect Ag presentation by a xenograft induces immunity in the absence of secondary lymphoid organs. These results have important implications for developing relevant xenotransplantation protocols.