We found that a homogenous CD4-/CD8- T cell bearing an invariant TCR encoded by V alpha 14J alpha 281 with a one-base N-region is highly dominant in the periphery (2-3% in spleen). Surprisingly, the high expression of this invariant V alpha 14 TCR is a general phenomenon in all laboratory strains irrespective of MHC haplotype and in some wild mouse subspecies. The majority of V alpha 14+ TCR is associated with J alpha other than J alpha 281 during the neonatal stage, after which the frequency of invariant V alpha 14J alpha 281 TCR expression increases with time reaching a maximum at around 5-8 weeks after birth. The dominant expression of V alpha 14J alpha 281 TCR is found in both euthymic and athymic mice. These results indicate that homogenous V alpha 14J alpha 281 T cells are positively selected in the periphery without thymic influence and that their VJ junction is important for positive selection. We also demonstrate that V alpha 14+ TCR gene rearrangements take place at extrathymic sites, such as bone marrow, liver, and intestine, since frequent nonproductive V alpha 14 TCR products and V alpha 14-J alpha 281 gene mediated signal sequences in circular DNA are detected as a result of TCR rearrangements in extrathymic tissues rather than in the thymus. This indicates the extrathymic development of V alpha 14J alpha 281 T cells. Furthermore, the biological roles of homogenous T cells bearing V alpha 14J alpha 281 TCR and the human counterpart of this invariant TCR are also discussed.