The ability to detect immunoglobulin-gene rearrangements has proved useful in confirming diagnoses of suspected B-cell lymphomas and in establishing their monoclonality. By analogy, we employed a cloned DNA probe for the beta chain of the T-cell receptor gene to determine whether gene rearrangements were present in human T-cell neoplasms representing various stages of T-cell development. Gene rearrangements were present in all cases of T-cell disorders except a single case of T gamma lymphocytosis, a disorder that has not been proved to be a clonal T-cell neoplasm. A germline gene configuration was present in all patients with non-T-cell neoplasms and in normal tissues from patients with T-cell lymphoma. The probe promises to be useful for confirming the pathological an immunologic diagnosis in difficult cases of T-cell disorders and for assessing the extent of disease.