To gain a better understanding of the organization of the complex T-cell antigen receptor alpha/delta (TCR alpha/delta) locus, a deletional analysis using the known six variable (V) regions of the TCR delta was performed in informative leukemic cell lines and fresh leukemias. We and others have previously reported a high incidence of V delta 2-(D)-D delta 3 rearrangements in non-T, non-B-lymphoid precursor acute lymphocytic leukemia (LP-ALL). In contrast V delta 4, V delta 5, V delta 6 rearrangements were rare or absent. V delta-J alpha rearrangements were found in LP-ALL and in T-ALL. Our deletion and rearrangement data combined with that of others suggest the following 5' to 3' organization of the TCR alpha/delta locus: V delta 6-(V delta 4-V alpha 1.2)-V alpha 12.1-V alpha 13.1-V delta 1-V delta 17.1-V delta 5-delta Rec-V delta 2-D/J/C delta-V delta 3-TEA-psi J alpha-J alpha G. The frequency of rearrangements of the various V delta genes suggests preferential use of the V delta most proximal to D/J delta.