The intra-thymic differentiation of functionally and phenotypically distinct T cell subsets, as well as their interaction with thymic epithelium, has been examined. Studies on thymocytes from genetically defective scid mice have suggested that TCR+ cells play a critical role in promoting the entry of thymocytes into the CD4/CD8 differentiation pathway as well as in promoting the maturation and organization of thymic medullary epithelium. The role of TCR expression by the differentiating thymocytes themselves was studied by using scid mice possessing an already rearranged TCRVP transgene which allows development of mature T cells. It was found that T cells at different stages of development expressed structurally distinct TCR complexes and that full differentiation into mature single-positive T cells requires surface expression of fully assembled TCR complexes. Early thymocyte differentiation was studied by in vivo and in vitro analysis of requirements for the transition of precursor thymocytes to CD4+CD8+ double positive cells. It was found that: a) negative selection can occur before the CD4+CD8+ stage of differentiation; b) cross-linking of TCR molecules on precursor thymocytes blocks their further differentiation by elimination of messenger RNA's encoding the co-receptor molecules CD4 and CD8 and the recombination activating genes l and 2; c) TCR-induced post-transcriptional regulation was specific for selective messenger RNAs, required protein synthesis, and was developmentally regulated; and d) the rate at which CD4+CD8+ thymocytes are generated in the thymus is controlled by TGF-beta.