Thymic development of T lymphocytes progresses as a consequence of both TCR-mediated and non-TCR-mediated interactions between thymocytes and stromal cells. As relB-deficient mice appear to lack thymic medullary epithelium and mature dendritic cells, we studied the effect of this "cortex-only" thymus on T cell development. Two major consequences were observed. First, in both relB mutant and TCR transgenic/relB mutant mice, positive selection of both TCR alpha beta and delta gamma T cells appeared to proceed normally, with export of fully functional T cells to the periphery, suggesting that the thymic medullary stromal cells are not required for full maturation of T cells nor is an organized medullary compartment required for accumulation of mature single positive CD4 and CD8 T cells. Second, thymic negative selection was impaired, as evidenced by significant autoreactive proliferative responses to normal spleen stimulators. Peripheral T cells in relB mutant mice showed an unusually high proportion of CD69+ and CD44high cells. While some of these cells may be autoreactive T cells, most of the cells appeared to be activated by cytokines produced by relB mutant nonlymphoid cells, as the effect is minimized in relB mutant bone marrow chimeras. In sum, while the TCR-mediated steps in T cell maturation require both thymic cortex and medulla (epithelium and dendritic cells) for normal positive and negative selection of the repertoire, non-TCR-mediated interactions in the thymic cortex alone are sufficient to generate mature functional T cells.