We have found recently that Graves' immunoglobulins (Igs) are more active under NaCl-free conditions to increase iodide uptake in FRTL-5 thyroid cells. The present study was designed to examine the effect of blocking-type thyrotropin (TSH)-receptor antibodies (TR-abs) in the same assay. FRTL-5 cells were incubated with crude Ig fractions from 13 patients with primary hypothyroidism having blocking TR-abs and 100 mU/l bovine TSH in NaCl-free Hanks' balanced salt solution (HBSS) supplemented with 274 mmol/l sucrose (HBSS(-)-sucrose) to keep isotonicity for 3 days, followed by the determination of 60 min of 125I uptake in the cells. Thyrotropin alone increased the uptake to 3.3- to 5.2-fold of the basal. When the highest concentration (250 ml serum equivalent (eq.)/l) of the Ig was used, the TSH-stimulated 125I uptake was decreased to 0-42% in 11 of the 13 cases. When the lower concentrations (0.4-50 ml serum eq./l) were used, however, 125I uptake was enhanced (1.4- to 11.4-fold) unexpectedly in all 13 cases. Such an enhancement was specific to blocking TR-abs, because the effect was duplicated by the purified IgG but not by Igs from 10 healthy control subjects or from three TR-ab-negative hypothyroid patients. When Graves' Igs, instead of TSH, were used as a stimulator, the similar stimulatory effect of the Ig with blocking activity was observed on the 125I uptake induced by three Graves' Igs but not on the response to one Graves' Ig. These Igs alone displayed no stimulating activity. When isotonic 5H medium was used instead of HBSS(-)-sucrose, effects of these 13 Igs were only inhibitory on the TSH-stimulated 125I uptake.(ABSTRACT TRUNCATED AT 250 WORDS)