TSH preincubation induces refractoriness to further stimulation of cAMP accumulationby the hormone in cultured thyroid cells. The question of whether the thyroidstimulating antibodies (TSAb) present in sera of patients with Graves’ disease have the same desensitizing effect as TSH was evaluated in this study using a differentiated strain of rat thyroid cells (FRT-L5) which requires TSH for growth and develops refractoriness to acute TSH stimulation of cAMP accumulation. Cells cultured for 4-5 days in medium deprived of TSH recovered responsiveness to TSH and TSAb. Refractoriness to TSH was reinduced in these cells by a 24-h preincubation with 250 µU/ml TSH. The addition of 100 µKI or 100 µM methimazole to the medium togetherwith TSH did not modify TSH-induced refractoriness. Four different TSAb preparations all induced refractoriness to TSH-stimulated cAMP accumulation. TSAb preincubation induced refractoriness to acute TSAb as well as TSH-stimulated cAMP accumulation. TSAb-induced desensitization was dose dependent and required prolonged (8–24 h) exposure to TSAb. Furthermore, the desensitization induced by TSAb as well as that caused by TSH were significantly inhibited by cycloheximide, indicating the need for de novo protein synthesis. In conclusion, TSAb mimics TSH in the induction of refractoriness in FRT-L5 cells.