To assess the abnormalities of insulin receptor in patients with Werner's syndrome, we established Epstein-Barr virus transformed lymphocytes and studied the binding as well as kinase activities of insulin receptor. The insulin binding to both intact cells and WGA-purified insulin receptor preparations was within normal range. Autophosphorylation of the beta-subunit was not altered in patients with Werner's syndrome, and the receptors of these patients phosphorylated an exogenous substrate to a degree comparable to that of the normal participants. Taken together, these findings indicate that insulin resistance in Werner's syndrome likely is caused by a defect that cannot be detected by means used in the present study.