Most of the diversity in T cell receptor subunits resides in the region that is the equivalent of the CDR3 of immunoglobulins. In order to learn more about the relative contributions of the various mechanisms that generate this diversity we have analyzed the sequences of alpha chain transcripts from BALB/c thymus. The J alpha repertoire of BALB/c mice was examined by comparison of new J alpha sequences and previously published sequences. Among the 41 J alpha genes examined, most of the diversity is located at the 5' end, consistent with the notion that this region contacts the antigen. VJ junctional diversity was examined by sequencing various V alpha J alpha combinations derived from different stages of development. Deletion of bases from the ends of V and J genes does not occur with equal frequency. A greater number of bases were deleted on average from the ends of J genes. Bases were added at junctions frequently in isolates from adult animals, consistent with the presence of terminal deoxynucleotidyl transferase. However, there were short stretches of sequences at junctions which were also present at the 5' end of J genes. These findings extend recent observations that alpha chain genes use multiple mechanisms for generating diversity.