During differentiation in the thymus, immature T cells progress through an ordered sequence of developmental stages that are best characterized by variable expression of the co-receptor molecules CD4 and CD8. Crosslinking of T cell receptor (TCR) molecules on precursor thymocytes was found to block their differentiation into CD4+CD8+ cells by eliminating messenger RNA's encoding two families of developmentally important molecules: the co-receptor molecules CD4 and CD8 and the recombination activating genes 1 and 2. TCR-induced post-transcriptional regulation in early thymocytes was specific for selective messenger RNA's, required protein synthesis, and was itself developmentally regulated. These data identify a post-transcriptional mechanism that is influenced by TCR signals and that regulates early thymocyte development.