The frequency of circulating alloreactive human memory T cells correlates with allograft rejection. Memory T cells may be divided into effector memory (T EM) and central memory (T CM) cell subsets, but their specific roles in allograft rejection are unknown. We report that CD4+ T EM (CD45RO+ CCR7− CD62L−) can be adoptively transferred readily into CB-17 SCID/bg mice and mediate the destruction of human endothelial cells (EC) in vascularized human skin grafts allogeneic to the T cell donor. In contrast, CD4+ T CM (CD45RO+ CCR7+ CD62L+) are inefficiently transferred and do not mediate EC injury. In vitro, CD4+ T EM secrete more IFN-γ within 48 h in response to allogeneic ECs than do T CM. In contrast, T EM and T CM secrete comparable amounts of IFN-γ in response to allogeneic monocytes (Mo). In the same cultures, both T EM and T CM produce IL-2 and proliferate in response to IFN-γ-treated allogeneic human EC or Mo, but T CM respond more vigorously in both assays. Blockade of LFA-3 strongly inhibits both IL-2 and IFN-γ secretion by CD4+ T EM cultured with allogeneic EC but only minimally inhibits responses to allogeneic Mo. Blockade of CD80 and CD86 strongly inhibits IL-2 but not IFN-γ production by in response to allogeneic EC or Mo. Transduction of EC to express B7-2 enhances allogeneic T EM production of IL-2 but not IFN-γ. We conclude that human CD4+ T EM directly recognize and respond to allogeneic EC in vitro by secreting IFN-γ and that this response depends on CD2 but not CD28. Consistent with EC activation of effector functions, human CD4+ T EM can mediate allogeneic EC injury in vivo.