The diversity of the T cell receptor repertoire is generated by rearrangement of gene elements in immature thymocytes. To identify a thymic signal that induces this rearrangement, a variety of agents were tested for their ability to induce rearrangement of the T cell receptor β gene in suspensions of thymocytes from mouse embryos at day 14 of gestation. Of 16 agents tested, only interleukin-7 (IL-7) induced V(D)J gene rearrangement and sustained expression of the RAG-1 and RAG-2 genes, which are known to control rearrangement. These data implicate IL-7, a cytokine that is abundantly expressed in embryonic thymus, in driving gene rearrangement during early T cell development.