Rearrangement of the immunoglobulin (Ig) and T-cell receptor (TcR) genes generally has been considered a useful marker of B- and T-cell lineage in lymphoproliferative disorders. However, concomitant rearrangements of Ig and TcR genes have been commonly reported in the most immature lymphoid malignancies, mainly in B-cell precursor acute lymphoblastic leukemia (ALL). To better characterize the nature of this lineage promiscuity, we have analyzed the configuration of the TcR delta locus in 75 B-precursor ALL. The large majority of cases (87%) displayed a rearrangement or deletion at the delta locus. Among the 57 nondeletional rearrangements, two patterns were predominant and both appeared to derive from partial joining: a V delta-(D)-D delta 3 (32/57) and a D delta 1/2-D delta 3 (11/57) type. A single V delta gene (V delta 2) appeared to be involved in the first type of rearrangement. These findings are at variance with T-ALL, where rearrangements 5' to V delta 2 are usually found. It remains to be elucidated whether this incomplete attempt of V delta 2 gene assembly is related to the neoplastic event or represents a physiologic predisposition occurring during early stages of the normal lymphocyte differentiation.