We used Southern blot analysis to investigate the T cell receptor and immunoglobulin (Ig) genes in tumor DNA derived from 13 patients with acute lymphoblastic leukemia (ALL). Rearrangement of both alleles of the T cell receptor beta chain gene was demonstrated in the three individuals with T cell ALL, while the Ig genes remained in germline configuration. In contrast, in nine of ten patients with non-T, non-B ALL (seven common ALL antigen [CALLA]-positive), Ig genes were rearranged while the T cell receptor genes were intact: the noteworthy exception was a CALLA-positive individual in whom one allele of the T cell receptor was rearranged and in whom a minor rearranged Ig band was detected as well. This exception indicates that CALLA-positive non-T, non-B ALL includes proliferations that have undergone an initial genetic step toward T cell differentiation. With probes now available for both the Ig and T cell receptor genes, analysis of genomic tumor DNA is useful in all variants of ALL.