We studied the suitability of using the recombinant human TSH receptor expressed in Chinese hamster ovary cells in a TSH binding inhibition (TBI) assay for autoantibodies against the TSH receptor. Purified immunoglobulin G (IgG) containing potent thyroid-stimulating immunoglobulin bioactivity competed for radiolabeled TSH binding to recombinant TSH receptor in parallel to inhibition by unlabeled TSH. Using polyethylene glycol-prepared IgG, this assay discriminated very well between sera from normal individuals [TBI, 0.98 +/- 0.04 (+/- SD); range, 0.92-1.08; n = 35] and patients with untreated Graves' disease (TBI, 0.49 +/- 0.23; range, 0.06-0.98; n = 93). Only four of the sera from the untreated Graves' patients were TBI negative (greater than or equal to 0.92), providing a sensitivity of 96%. In sera from patients with Graves' disease receiving antithyroid drug therapy, TBI was 0.63 +/- 0.18 (range, 0.19-0.97; n = 75). In this group of treated patients, 2 of 75 were TBI negative. Four of 12 patients with Hashimoto's thyroiditis (33%) were positive for TBI activity. All 18 patients with nonautoimmune thyroid diseases (toxic nodular goiter, single toxic adenoma, subacute thyroiditis, or thyroid cancer) were TBI negative. Correlation of the TBI values with thyroid-stimulating immunoglobulin bioactivity revealed a generally positive correlation (r = 0.31; P less than 0.05); however, there were many discrepancies among individual sera. TSI bioactivity was undetectable in all 4 patients with Hashimoto's thyroiditis who were TBI positive. These data indicate that the recombinant TSH receptor in Chinese hamster ovary cells is a suitable system for detecting TBI activity in the sera of patients with autoimmune thyroid disease.