To explore whether and how T cells can affect myelopoiesis, we investigated myeloid differentiation in a model for T cell-mediated immune activation. We found that CD70-transgenic (CD70TG) mice, which have elevated numbers of interferon-γ (IFN-γ)-producing effector T cells in the periphery and bone marrow, are almost devoid of eosinophilic granulocytes. Induction of allergic airway inflammation in these mice failed to induce eosinophilia as well as airway hyperresponsiveness. CD70TG mice also have strongly reduced numbers of eosinophil lineage-committed progenitors, whereas granulocyte/macrophage progenitors from these mice are unable to generate eosinophils in vitro. We found that granulocyte/macrophage progenitors express IFN-γR1 and that IFN-γ is sufficient to inhibit eosinophil differentiation of both murine and human progenitor cells in vitro. We demonstrate that inhibition of eosinophil development in CD70TG mice is IFN-γ-dependent and that T cell-derived IFN-γ is sufficient to inhibit eosinophil formation in vivo. Finally, we found that IFN-γ produced on anti-CD40 treatment and during viral infection can also suppress eosinophil formation in wild-type mice. These data demonstrate that IFN-γ inhibits the differentiation of myeloid progenitors to eosinophils, indicating that the adaptive immune system plays an important role in orchestrating the formation of the appropriate type of myeloid cells during immune activation.