The organization and expression of the beta chain of T cell antigen receptor gene (beta-TCR) and Ig H and L chain genes were analyzed by Southern blot technique in 24 patients with a diagnosis of acute myeloblastic leukemia (AML). Rearrangements of the beta-TCR genes were seen in DNA samples from 3 of the 24 patients. One of these three patients also showed rearrangement of the Ig H chain gene. RNA samples from all three patients expressed a beta-TCR gene transcript on dot blot analysis. However, on Northern blot analysis, one patient expressed an incomplete 1.0 kb transcript and no Ig H chain mRNA, despite a rearranged configuration. The karyotypes of two of these patients showed abnormalities involving chromosome 7. Rearrangements of T cell antigen receptor genes may occur in nonlymphoid malignancy, and is consistent with the concept of lineage infidelity in AML.