Blast formation and mitotic activation of G0-arrested mouse thymocytes were triggered by the addition of concanavalin A plus interleukin 2 (IL-2) to the culture medium. When added alone, Con A induces within 6 hr a complex reprogramming ("priming") that comprises the activation of the IL-2 receptor gene. The primed thymocytes are competent to interact with IL-2 and to respond to its growth-promoting effect, which corresponds to blast formation and mitotic activation. Cyclosporin A, an immunosuppressive cyclic peptide of fungal origin, prevents in T lymphocytes the activation of a set(s) of genes encoding lymphokines and the IL-2 receptor but does not affect their expression once they have been activated. The biomedical implications of these observations are discussed.