Rearrangements of the immunoglobulin heavy chain joining region (JH) genes occur in some T lymphocytes, probably because the mechanism for assembly of T-cell receptor encoded genes is very similar to that for immunoglobulins. Two such rearrangements described previously represented proper fusion of a DH and JH gene. We have cloned and analyzed the JH rearrangements found on both alleles in the T lymphoma ST4. One represents conventional recombination between a member of the SP2 DH family and JH3. On the other allele, JH4 has recombined with a sequence within the DH locus but not at a DH gene. This recombination involved the heptamer but not the nonamer of the bipartite recognition signal required for DH-JH joining. This result suggests that the heptamer may be the primary determinant of the specificity in V-gene assembly and that the DH locus as a whole may be a preferred target for recombination.