The precise mechanisms that govern the commitment of CD4 T cells to become Th1 or Th2 cells in vivo are incompletely understood. Recent experiments demonstrate colocalization of the IFN-γR chains with the TCR during activation of naive CD4 T cells, suggesting that association of these molecules may be involved in determining lineage commitment. To test the role of IFN-γ and its receptor in the generation of Th1 Ag-specific CD4 T cells, we analyzed mice after infection with Listeria monocytogenes or lymphocytic choriomeningitis virus. In the absence of IFN-γ, Ag-specific CD4 T cells were generated in response to both these infections. In addition, IFN-γ-producing (Th1) Ag-specific CD4 T cells were generated in mice lacking the ligand-binding chain of the IFN-γR (IFN-γR1−/−) or the signaling chain (IFN-γR2−/−). There was no increase in the number of IL-4-producing Ag-specific CD4 T cells, nor was there a decrease in the expression of T-bet in the absence of functional IFN-γ signaling, indicating that the cells were committed Th1 cells. Thus, both chains of the IFN-γR are dispensable for the generation of Th1 Ag-specific CD4 T cells after infection in vivo.