SummaryThe results of precipitin and flocculation tests on the serum of 76 patients with thyroid disease are presented and discussed. A positive precipitin reaction to thyroid extract indicates that, at one time or other, the patient has become sensitized to thyroglobulin.A thyroid precipitin test employing normal electrophoretic equipment is described. The test is slightly more sensitive, simpler and more rapid than the agar diffusion test. Taken in conjunction with clinical assessment and other available investigations, this precipitin test is a valuable diagnostic procedure.