Full activation of naive CD8 T cells requires Ag, costimulation, and a third signal that can be provided by IL-12. Brief exposure (6 h) to Ag and B7-1 is sufficient to stimulate multiple rounds of cell division, but clonal expansion and development of effector function are minimal even when signal 3 is present. Full activation instead requires concerted signaling by Ag, B7-1, and IL-12 for greater than 40 h. Thus, the gene expression program required for cell division can be initiated by brief interaction with Ag and costimulation, but maintaining the expression of the genes needed for survival and effector function requires prolonged signaling by a signal 3 cytokine in concert with Ag and costimulation.