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Costimulatory interactions between CD28 and the B7 family have been shown to augment T cell responses in general. To further assess the importance of the costimulatory signals generated through CD28, the allogeneic response was examined in CD28-deficient mice. T cells from CD28-deficient mice showed reduced proliferation and cytokine production in response to allogeneic stimulator cells in vitro. However, CD28-deficient T cells developed cytotoxic activity against allogeneic target cells in vitro and efficiently rejected skin allografts in vivo. These results suggest that the costimulatory signals through CD28 are not essential for the induction of alloreactive effector functions in vitro or in vivo.