beta and gamma T cell receptor (TCR) gene configuration was studied in 12 patients with large granular lymphocyte T cell leukemia (LGL-leukemia). Both genes were found rearranged in ten cases. In the remaining two patients TCR beta was found in germline configuration. In one of them rearrangement of T cell-rearranging gene gamma (TRG gamma) and a gamma mRNA were demonstrated. We suggest that in this patient the leukemic T cells arose from one of the rare T cells bearing a gamma-delta rather than an alpha-beta TCR heterodimeric molecule. In the other patient several discrete TRG gamma rearrangements were detected. Because her leukemic cells were shown to be monoclonal on the grounds of their karyotype, we suggest that her leukemia originated before any rearrangement had taken place. The combined use of TCR beta and TRG gamma probes provides new information on the origin and clonal expansion of lymphoid cells in LGL-leukemia.