Reperfusion of organ allografts induces a potent inflammatory response that directs rapid memory T cell, neutrophil, and macrophage graft infiltration and their activation to express functions mediating graft tissue injury. The role of cardiac allograft IL-1 receptor (IL-1R) signaling in this early inflammation and the downstream primary alloimmune response was investigated. When compared with complete MHC-mismatched wild-type cardiac allografts, IL-1R−/− allografts had marked decreases in endogenous memory CD8 T cell and neutrophil infiltration and expression of proinflammatory mediators at early times after transplant, whereas endogenous memory CD4 T cell and macrophage infiltration was not decreased. IL-1R−/− allograft recipients also had marked decreases in de novo donor-reactive CD8, but not CD4, T cell development to IFN-γ–producing cells. CD8 T cell–mediated rejection of IL-1R−/− cardiac allografts took 3 wk longer than wild-type allografts. Cardiac allografts from reciprocal bone marrow reconstituted IL-1R−/−/wild-type chimeric donors indicated that IL-1R signaling on graft nonhematopoietic-derived, but not bone marrow–derived, cells is required for the potent donor-reactive memory and primary CD8 T cell alloimmune responses observed in response to wild-type allografts. These studies implicate IL-1R–mediated signals by allograft parenchymal cells in generating the stimuli-provoking development and elicitation of optimal alloimmune responses to the grafts.