Introduction of TCR alpha transgene, TCR beta transgene, or both into RAG-2-/-mice differentially rescues T cell development. RAG-2-/- mice have small numbers of TCR-CD4-CD8-(double negative, DN) thymocytes that express CD3 gamma delta epsilon and zeta proteins intracellularly. Introduction of a TCR beta transgene, but not a TCR alpha transgene, into the RAG-2-/- background restored normal numbers of thymocytes. These cells were CD4+CD8+ (double positive, DP) and expressed small amounts of surface TCR beta chain dimers in association with CD3 gamma delta epsilon but not zeta. RAG-2-/- mice that expressed alpha and beta TCR transgenes developed both DP and single positive thymocytes. Thus, the TCR beta subunit, possibly in association with a novel CD3 complex, participates in the DN to the DP transition.