Exclusion and inclusion of T cell receptor (TCR) genes were analyzed in a6 TCR transgenic mice. Both transgenes are expressed unusually early on the surface of CD4-8-, HSA+, IL-2R thymocytes. These progenitor cells give rise to progeny, which at the singlecell level contains endogenous a but not f3 TCR-RNA as well as protein, in addition to products encoded by the transgenes. Thus, the surface expression of an a8 TCR does not prevent further a TCR rearrangement in immature thymocytes that still transcribe RAG-1 and RAG-2 genes. Reduced levels of RAG-1 and RAG-2 RNA are detectable only in CD4+ 8+ TCRh’@’cells, which result from positive selection in the thymus. The results suggest that a developing T cell may try different a8 TCRs for binding to thymic MHC ligands, and that recombination at the a locus ceases only after positive selection.