In G₀-arrested mouse thymocyte cultures the combination of concanavalin A and interleukin-2 ([Con A+IL-2]) induced in 35,5% of the cells a mitotic reaction (blastic transformation). We report results on the sequence of molecular events induced by [Con A+IL-2] or by Con A alone. Taking into account recent observations with cyclosporin A, an immmosupressive drug, we were led to the conclusion that the mitotic reaction in mouse thynocytes occurs in two distinct phases: phase 1 corresponding to a very early stimulation of hnRNA and protein synthesis and phase 2 to all subsequent events of blastic transformation.