How cytokines control differentiation of helper T (T_H) cells is controversial. We show that T-bet, without apparent assistance from interleukin 12 (IL-12)/STAT4, specifies T_H1 effector fate by targeting chromatin remodeling to individual interferon-γ (IFN-γ) alleles and by inducing IL-12 receptor β2 expression. Subsequently, it appears that IL-12/STAT4 serves two essential functions in the development of T_H1 cells: as growth signal, inducing survival and cell division; and as trans-activator, prolonging IFN-γ synthesis through a genetic interaction with the coactivator, CREB-binding protein. These results suggest that a cytokine does not simply induce T_Hfate choice but instead may act as an essential secondary stimulus that mediates selective survival of a lineage.