Abstract: A fast growing, continuous, functional rat thyroid cell strain, FRTL-5, which maintains functional characteristics of iodide uptake and thyroglobulin synthesis over prolonged periods of culture is cloned from FRTL cells obtained from primary cultures of Fischer rat thyroid glands. The FRTL-5 cells are cultured in a medium containing approximately 5 percent calf serum supplemented with a mixture of hormones, at least one of which is thyrotropin.The FRTL-5 cells are employed in a series of assays which measure thyroid stimulatory or inhibitory factors. The FRTL-5 system of assay specifically measures thymidine incorporation, cAMP elevation and iodide uptake and permits the evaluation of patient sera, particularly those afflicted with Graves' disease and other autoimmune thyroid diseases, thereby providing a means for determining appropriate methods of treatment.