It is widely accepted that females have superior immune responses than males, but the ways by which sex hormones may enhance T cell responses are still poorly understood. In the present study,we analyzed the effect of estrogens on CD4 T cell activation and differentiation after immunization with exogenous antigens. We show that administration of low doses of 17ß-estradiol (E2) to castrated female mice results in a striking increase of antigen-specific CD4 T cell responses and in the selective development of IFN-γ-producing cells. Quantitative assessment of the frequency of T cells bearing a public TCR ß chain CDR3 motif demonstrated that the clonal size of primary antigen-specific CD4 T cells was dramatically increased in immune lymph nodes from E2-treated mice. By usingmice with disrupted estrogen receptor (ER) α or ß genes, we show that ERα, but not ERβ, was necessary for the enhanced E2-driven Th1 cell responsiveness. Furthermore, ERα expressionin hematopoietic cells was essential, since E2 effects on Th1 responses were only observed in mice reconstituted with bone marrow cells from ERα+/+, but not ERα-deficient mice. These results demonstrate that estrogen administration promotes strong antigen-specific Th1 cell responses in a mechanism that requires functional expression of ERα in hematopoietic cells.