Two primary types of TCR-alpha/beta+ T cells are found in the peripheral lymphoid system; CD4+8- T cells, with MHC-class II restricted TCR, and CD4-8+ T cells, which are MHC-class I restricted. Both lineages develop in the thymus from a series of common precursors. However, the precise stage at which they diverge, and the combination of factors that regulates such divergence, are not well defined. The up-regulation of CD3/TCR to high mature levels is thought to be an early event associated with positive selection for self-MHC recognition. Using purified cells from bcl-2 transgenic mice in order to overcome the limitations imposed by cell death on normal thymocytes, we find that a minor subset of CD4+8+ thymocytes expressing high levels of CD3/TCR gives rise to both CD4+8- and CD4-8+ mature cells upon intrathymic transplantation, but only to CD4-8+ in culture. Thus, in addition to demonstrating the dual lineage potential of this subset, these findings show that additional post-selection processing events are required for the production of mature thymocytes, and that CD4+8- and CD4-8+ subsets differ in the types of processing required.