A 59-year-old woman with systemic lupus erythematosus was found to have marked hyperglycemia, extreme insulin resistance and abnormally high plasma immunoreactive insulin. Her circulating erythrocytes displayed a dramatic decrease of 125I-labeled insulin binding. Both the whole serum and purified IgG fraction strongly inhibited the binding of radiolabeled insulin to control erythrocytes. These results suggested, although indirectly, the existence of antibodies to insulin receptors in the serum of the patient. To directly investigate this issue, we used an enzyme-linked solid-phase immunoassay which allows the detection and enumeration of lymphocytes secreting antibodies towards insulin receptors. Peroxidase-conjugated anti-human immunoglobulin is used to reveal the binding of antibodies to insulin receptor-coated dishes. We demonstrated that the patient's mononuclear cells, when briefly incubated in Petri dishes with partially purified insulin receptor, were able to secrete immunoglobulins of G class specifically directed to the antigen. Moreover, only a fraction of the whole population of anti-insulin receptor antibodies was directed towards the insulin binding region of the receptor, seemingly corresponding to the auto-antibodies detected with conventional binding-inhibition assay.