The survival of T lymphocytes is tightly controlled during development. Here, we show that Bcl-x,, a protein homologue of Bcl-2, is highly regulated in the thymus in a pattern different than that ofBcl-2. The maximum expression was in CD4+ CD8+ thymocytes, a developmental stage where Bcl-2 is downregulated. To assess the role of Bcl-x, in thymocyte apoptosis, we generated mice overexpressing an EN,-bcl-x transgene within the T cell compartment. Constitutive expression of BC'-XL resulted in accumulation of thymocytes and mature T cells in lymphoid organs. Thymocytes overexpressing BC'-xL exhibited increased viability in vitro and were resistant to apoptosis induced by different signals, including glucocorticoid, y irradiation, calcium ionophore, and CD3 cross-linking. However, BC'-xL was unable to block clonal deletion of thymocytes reactive with self-superantigens or HY antigen. These studies demonstrate that Bcl-2 and BC'-XL, two functionally related proteins, are regulated independently during T cell development. In contrast to Bcl-2, which has been implicated in the maintenance of mature T cells, BC'-xL appears to provide a survival signal for the maintenance of more immature CD4+ CD8+ thymocytes before positive selection.