Both innate and adaptive immune cells are involved in the allograft response. But how the innate immune cells respond to allotransplants remains poorly defined. In the current study, we examined the roles of NK cells and macrophages in recognizing and rejecting allogeneic cells in vivo. We found that in naive mice NK cells are the primary effector cells in the killing of allogeneic cells via "missing self" recognition. However, in alloantigen-presensitized mice, NK cells are dispensable. Instead, macrophages become alloreactive and readily recognize and reject allogeneic nonself. This effect requires help from activated CD4(+) T cells and involves CD40/CD40L engagement, because blocking CD40/CD40L interactions prevents macrophage-mediated rejection of allogeneic cells. Conversely, actively stimulating CD40 triggers macrophage-mediated rejection in the absence of CD4(+) T cells. Importantly, alloantigen-primed and CD4(+) T cell-helped macrophages (licensed macrophages) exhibit potent regulatory function in vivo in an acute graft-versus-host disease model. Together, our data uncover an important role for macrophages in the alloimmune response and may have important clinical implications.