Recently, we demonstrated insulin resistance due to reduced glucose storage in young relatives of Type 2 diabetic patients. To investigate whether this was associated with a defective insulin receptor kinase, we studied ten of these young (27 +/- 1 years old) non-obese glucose tolerant first degree relatives of patients with Type 2 diabetes and eight matched control subjects with no family history of diabetes. Insulin sensitivity was assessed by a hyperinsulinaemic, euglycaemic clamp. Insulin receptors were partially purified from muscle biopsies obtained in the basal and the insulin-stimulated state during the clamp. Insulin binding capacity was decreased by 28% in the relatives (p < 0.05) in the basal biopsy. Tyrosine kinase activity in the receptor preparation was decreased by 50% in both basal and insulin-stimulated biopsies from the relatives. After stimulation with insulin "in vitro", kinase activity was reduced in the relatives in basal (p < 0.005) and insulin-stimulated (p < 0.01) biopsies and also when expressed per insulin binding capacity (p approximately 0.05). Insulin stimulation of non-oxidative glucose metabolism correlated with "in vitro" insulin-stimulated tyrosine kinase activity (r = 0.61, p < 0.01) and also when expressed per binding capacity (r = 0.53, p < 0.025). We suggest that the marked defect in tyrosine kinase activity in partially purified insulin receptors from skeletal muscle is an early event in the development of insulin resistance and contributes to the pathophysiology of Type 2 diabetes.