Recent studies using mouse models demonstrate that CD4+ T cells are sufficient to mediate acute cardiac allograft rejection in the absence of CD8+ T cells and B cells. However, the mechanistic basis of CD4-mediated rejection is unclear. One potential mechanism of CD4-mediated rejection is via elaboration of proinflammatory cytokines such as IFN-γ. To determine whether IFN-γ is a critical cytokine in CD4-mediated acute cardiac allograft rejection, we studied whether the expression of IFN-γ receptors on the donor heart was required for CD4-mediated rejection. To investigate this possibility, purified CD4+ T cells were transferred into immune-deficient mice bearing heterotopic cardiac allografts from IFN-γ receptor-deficient (GRKO) donors. While CD4+ T cells triggered acute rejection of wild-type heart allografts, they failed to trigger rejection of GRKO heart allografts. The impairment in CD4-mediated rejection of GRKO hearts appeared to primarily involve the efferent phase of the immune response. This conclusion was based on the findings that GRKO stimulator cells provoked normal CD4 proliferation in vitro and that intentional in vivo challenge of CD4 cells with wild-type donor APC or the adoptive transfer of in vitro primed CD4 T cells failed to provoke acute rejection of GRKO allografts. In contrast, unseparated lymph node cells acutely rejected both GRKO and wild-type hearts with similar time courses, illustrating the existence of both IFN-γ-dependent and IFN-γ-independent mechanisms of acute allograft rejection.