Rearrangements of the T cell gamma (T gamma) gene were studied in primary neoplastic cells from 75 patients with leukemia or lymphoma. T gamma gene rearrangements were observed in 19 of 21 T cell neoplasms; 14 of 21 immature B cell leukemias, including 4 out of 5 patients with rearrangements of both immunoglobulin heavy-chain (JH) and T cell receptor beta chain (T beta) genes; none out of 16 nonlymphoid leukemias. Thus, T gamma gene rearrangement is frequently found in immature B cells and is not always found in T cells showing T beta gene rearrangement, but it is not detected in nonlymphoid cells. Furthermore, T gamma gene rearrangement in cells with the germline configuration of the JH and T beta genes was observed. These results indicate that the detection of T gamma gene rearrangement does not allow a clear assignment to a particular lineage. However, an analysis of T gamma gene rearrangement provides a further potential tool to establish the lymphoid cellular origin and clonality of hematologic neoplasms and identify the normal stages of lymphocyte differentiation.