In this communication we described about simple method for measuring adenylate cyclase activity of human thyroid plasma membrane using competitive protein assay for C-AMP and cold ATP as substrate for the enzyme. Technical details for preparing the plasma membrane were also described. In our assay system for thyroid stimulator, 44 immunoglobulins from patients with Graves' disease and 15 normal immunoglobulins showed no stimulating activity on human thyroid plasma membrane adenylate cyclase except one immunoglobulin from patients with Graves' disease.