Both the t(14; 18) and the t(11; 14) chromosomal translocations are associated with human B-cell neoplasms. Sequence analysis of the regions on chromosomes 18q- and 11q- involved in the joining of chromosomes 14 and 18 and of chromosomes 11 and 14 during their respective translocations indicates the involvement of the DH region. There is a deletion of the region between the involved D segment and the JH region, and extranucleotides are present at the joining sites on the 18q- and on the 11q- chromosomes. We have previously reported that the joining region on chromosome 14q+ involved in both the t(11; 14) and the t(14; 18) translocations are structurally identical. We now report that the joining regions on chromosomes 11q- and 18q- involved in these two different translocations are also identical. Thus, these translocations arose by the same mechanisms. The deletions of the region between the involved segment and the JH region must likely occur during either a D-J recombination event on the 18q- and 11q- chromosomes following the translocations or, alternatively, during the D-J recombination event that results in the translocation.