Rearrangements of T cell receptor beta and gamma chain (T beta and T gamma) genes were analyzed by Southern blot method in samples from 30 patients with adult T cell leukemia (ATL) and 17 patients with non-ATL T cell neoplasms. The DNA probes used were the constant and joining region of T beta gene and the joining region of T gamma gene. Rearranged bands of T beta gene on one or both allelic chromosomes were detected in all neoplastic T cells, even those of smoldering ATL, in which only a small percentage of peripheral blood T cells were detected as leukemic. T gamma gene was rearranged in the cells of all but one patient, the exception being one ATL patient. In order to test whether any given variable region (V) of T beta gene was expressed in ATL cells, two functionally rearranged V beta sequences of ATL were compared with a V beta sequence from T cells acute lymphoblastic leukemia cells. No significant homologies were noted among the three deduced gene product amino acid sequences, confirming that T beta molecules of ATL cells contained no specific structures in common. The observed heterogeneity of T beta and T gamma gene rearrangements in ATL cells further supported these findings.