The variables that influence priming of human naive CD4+ T cells by dendritic cells (DC) were dissected in vitro by analyzing the response to the bacterial superantigen toxicshock syndrome toxin or to alloantigens. We show that under conditions that force DC-T cell interactions a single DC can prime up to 20 naive T cells. Moreover, the strength of antigenic stimulation, as determined by DC numbers, antigen dose, TCR avidity and duration of DC-T cell interactions, drives the progressive differentiation of proliferating T cells from a non-effector CCR7+ stage, to an effector CCR7- stage and, eventually, to cell death. We also show that the proliferating CCR7+ and CCR7- populations share clonotypic sequences, demonstrating that the two cell fates can be generated within a single clone. Taken together these results indicate that the strength of antigenic stimulation regulates T cell progression through thresholds of proliferation, differentiation and death. However, the random nature of DC-T cell encounters introduces a critical stochastic element in T cell stimulation, which leads to the generation of cells endowed with distinct homing potentials and effector functions within a given T cell clone.