Single cell studies have identified intraclonal heterogeneity of cytokine production by activated T cells. To investigate implications of cytokine heterogeneity for cell fate, an interleukin (IL)-2 promoter-green fluorescent protein (GFP) reporter transgenic model was developed to track IL-2 + and IL-2 - T cells during differentiation from naive precursors. Antigen-activated IL-2 + and IL-2 - cells had comparable proliferative capacities in primary responses. However, T cells that expressed IL-2 in primary responses demonstrated enhanced antigenic sensitivity and increased expression of effector cytokines in secondary responses in vitro and in vivo. Thus, heterogeneity of activation during a primary response translates into heterogeneous secondary responses, in which enhanced memory/effector function is linked to cells that previously exceeded an activation threshold associated with IL-2 gene transcription.