INSULIN-DEPENDENT diabetes mellitus (IDDM) is a T-cell-mediated autoimmune disease whose onset is believed to be triggered by unknown environmental factors acting on a predisposing genetic background. Islet-infiltrating T (IIT) cells from two IDDM patients, who had died at the onset of the disease from brain swelling as a complication of ketoacidosis, were analysed. The results provided evidence for the involvement of a pancreatic islet cell membrane-bound superantigen^1,2 as a diabetes aetiopatho-genetic factor. There was a selective expansion of a T-cell receptor (TCR) variable segment of the β-chain (Vβ7) in these IIT cells in association with unselected Vα-chain segments; extensive junc-tional diversity of the TCR Vβ7 chains; and evidence of positive selection, after exposure to diabetic islet cell membrane preparations, of Vβ7⁺ T-cell clones among peripheral blood lymphocytes from non-diabetic individuals.