The transcription factor GATA-3 is essential for early T cell development and differentiation of naive CD4+ T cells into Th2 effector cells. To study the function of GATA-3 during T cell-mediated immune responses in vivo, we investigated CD2-GATA3-transgenic mice in which GATA-3 expression is driven by the CD2 locus control region. Both in the CD4+ and the CD8+ T cell population the proportion of cells exhibiting a CD44 high CD45RB low CD62L low Ag-experienced phenotype was increased. In CD2-GATA3-transgenic mice, large fractions of peripheral CD4+ T cells expressed the IL-1 receptor family member T1/ST2, indicative of advanced Th2 commitment. Upon in vitro T cell stimulation, the ability to produce IL-2 and IFN-γ was decreased. Moreover, CD4+ T cells manifested rapid secretion of the Th2 cytokines IL-4, IL-5, and IL-10, reminiscent of Th2 memory cells. In contrast to wild-type CD4+ cells, which lost GATA-3 expression when cultured under Th1-polarizing conditions, CD2-GATA3-transgenic CD4+ cells maintained expression of GATA-3 protein. Under Th1 conditions, cellular proliferation of CD2-GATA3-transgenic CD4+ cells was severely hampered, IFN-γ production was decreased and Th2 cytokine production was increased. Enforced GATA-3 expression inhibited Th1-mediated in vivo responses, such as Ag-specific IgG2a production or a delayed-type hypersensitivity response to keyhole limpet hemocyanin. Collectively, these observations indicate that enforced GATA-3 expression selectively inhibits Th1 differentiation and induces Th2 differentiation. The increased functional capacity to secrete Th2 cytokines, along with the increased expression of surface markers for Ag-experienced Th2-committed cells, would argue for a role of GATA-3 in Th2 memory formation.