Insulin receptors (IR) are usually measured on the basis of their interaction with ¹²⁵I-labeled insulin, and although this method permits quantification of IR in human peripheral monocytes, it has the drawback of requiring a large volume of blood. In the present study, we characterized IR in human monocytes by flow cytometry (FCM) using indirect immunofluorescence with a murinemonoclonal antibody against human IR. The monocyte IR level was expressed as cumulative fluorescence intensity (CFI) and obtained using the formula: mean fluorescence intensity × positivecells (%)÷100. Before performing FCM, mononuclear cells were exposed to a saturating amount (100 μg/mι) of monoclonal anti-IR antibody, followed by incubation with fluorescein-conjugated sheep anti-mouse IgG. There was a good correlation between monocyte IR levels detected by FCM and by the usual method using ¹²⁵I-insulin binding. In order to assess the practical value of the present FCM method, we evaluated IR in monocytes from patients with Graves' disease, a pathological state well known to be associated with insulin hypersecretion. The IR level of monocytes in hyperthyroid patients was significantly lower than in normal subjects. The observation that there was an inverse correlation between fasting serum insulin level and IR expression by monocytes in Graves' disease suggests that down-regulation of IR was produced by hyperinsulinemia. Thus, thepresent FCM technique employing only a small volume of blood provides a new and convenient approach to the evaluation of IR which should allow more extensive clinical studies of the dynamics of this receptor in diabetes.