We previously found that the peripheral blood (PB) mononuclear cells (MCs) (PBMCs) of a patient with chronic neutropenia contained an expanded population of cytotoxic CD8+ T cells using a variable (V) region δ1 gene product in the T-cell receptor-α (TCR-α) polypeptide [Vδ1-constant(C)α+ T cells]. Sequencing of polymerase chain reaction (PCR) amplification products have now revealed a productive VδS1/joining (J)αIGRJa03/Cα rearrangement of the TCR-α gene, predominantly associated with a Vβ16/Dβ2.1/Jβ2.1/Cβ2 TCR-β gene, in these cells. Furthermore, we detected a markedly deficient proliferative response of the patient PBMCs to triggering with monoclonal antibodies (MoAbs) to the CD3 molecule, contrasting with a substantial response to the Vβ3, 12, 14, 15, 17 and 20-specific staphylococcal enterotoxin B (SEB) superantigen, suggesting defective TCR-mediated activation of the Vδ1+/Vβ16+ clone. Moreover, whereas triggering of Vδ1- T cells cultured with interleukin-2 (IL-2) by MoAb to the CD3 molecule enhanced proliferation, Vδ1-Cα+ T cells were inhibited by MoAbs to either CD3 or Vδ1. Vδ1-Cα+ T-cell clones spontaneously secrete interferon-γ (IFN-γ) and were further induced to release tumour necrosis factor (TNF-α) when triggered by anti-CD3 plus phorbol ester. Aberrant signalling by the clonotypic TCR together with the functional properties of the CD8+ Vδ1+/Vβ16+ clone may thus contribute to the immunohaematological abnormalities observed in this patient.