Deficient suppression of T cell responses to allergen by CD4 + CD25 + regulatory T cells has been observed in patients with allergic disease. Our current experiments used a mouse model of airway inflammation to examine the suppressive activity of allergen-specific CD4 + CD25 + T cells in vivo. Transfer of ovalbumin (OVA) peptide–specific CD4 + CD25 + T cells to OVA-sensitized mice reduced airway hyperreactivity (AHR), recruitment of eosinophils, and T helper type 2 (Th2) cytokine expression in the lung after allergen challenge. This suppression was dependent on interleukin (IL) 10 because increased lung expression of IL-10 was detected after transfer of CD4 + CD25 + T cells, and regulation was reversed by anti–IL-10R antibody. However, suppression of AHR, airway inflammation, and increased expression of IL-10 were still observed when CD4 + CD25 + T cells from IL-10 gene–deficient mice were transferred. Intracellular cytokine staining confirmed that transfer of CD4 + CD25 + T cells induced IL-10 expression in recipient CD4 + T cells, but no increase in IL-10 expression was detected in airway macrophages, dendritic cells, or B cells. These data suggest that CD4 + CD25 + T cells can suppress the Th2 cell–driven response to allergen in vivo by an IL-10–dependent mechanism but that IL-10 production by the regulatory T cells themselves is not required for such suppression.