Jak3, a member of the Janus family of tyrosine kinases, is essential for signaling via the receptors for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. These Jak3-dependent cytokines primarily activate STAT5 and are critical for lymphoid generation and differentiation. Using naïve CD4⁺ T cells from Jak3-deficient mice and wild type CD4⁺ T cells treated with a pharmacological inhibitor of Jak3, we report that Jak3-dependent cytokine signals are not required for the proliferation of naïve CD4⁺ T cells. This is illustrated by the similar percentage of divided cells, comparable cell divisions, intact cell cycle progression and unaffected regulation of cell cycle proteins in the absence of Jak3. In contrast to proliferation, differentiation of naïve CD4⁺ T cells into Th1 effector cells requires Jak3-dependent cytokine signals. In the absence of Jak3, naïve CD4⁺ T cells proliferate robustly, but produce little IFN-γ after Th1 polarization in vitro. This defect is not due to reduced activation of STAT1 or STAT4, nor to impaired up-regulation of the transcription factor T-bet. Instead, we find that T-bet binding to the Ifng promoter is greatly diminished in the absence of Jak3-dependent signals, correlating with a decrease in Ifng promoter accessibility and histone acetylation. These data indicate that while Jak3-dependent signals are dispensable for naïve CD4⁺ T cell proliferation, Jak3 regulates epigenetic modification and chromatin remodeling of the Ifng locus during Th1 differentiation.