DNA containing breakpoints of two different t(7;9) chromosomal translocations was cloned from the T lymphoblastic tumor cell lines SUP-T1 and SUP-T3. Sequence analysis of DNA from the t(7;9)(q34;q34.3) translocation of SUP-T1 revealed that chromosome 9 DNA had recombined with DNA 5' to rearranged D-J regions in the beta T cell receptor gene of chromosome 7. Restriction analysis and hybridization studies using DNA fragments cloned from the t(7;9)(q34;32) translocation of SUP-T3 confirmed that beta T cell receptor DNA is also joined to the DNA of chromosome 9 in these cells. Using hybridization probes for the two breakpoints, several other cases of T lymphoblastic tumors were shown to possess DNA rearrangements near the 9q34.3 and 9q32 sites. Hybridization with the 9q34.3 probe detected multiple transcripts in SUP-T1 RNA and small amounts of larger transcripts in T cells lacking the t(7;9)(q34;q34.3) translocation. This work directly demonstrates that the beta T cell receptor locus may frequently be involved in chromosomal translocations within T lymphoblastic neoplasms.