We previously developed a radioreceptor assay which is presumably specific for detection of blocking-type anti-TSH receptor (TSHR) antibodies using unsolubilized porcine TSHR. Employing this assay, we measured blocking TSH-binding inhibitory immunoglobulin (TBII) in the sera from 30 untreated Graves' patients and compared the results with those of a bioassay measuring thyroid stimulation blocking antibodies (TSBAb). Blocking TBII was positive in 9 of 30 sera (30%) and the blocking TBII activity was correlated with the total TBII value, which was measured by the radioreceptor assay using solubilized porcine TSHR. On the other hand, TSBAb determined with the conventional bioassay was positive in only 2 sera (6.7%), and no correlation was observed with the TSAb activity. In some cases in which the TSAb activity was rather high, TSBAb could not be detected by bioassay, whereas blocking TBII was positive. There was no correlation of blocking TBII activity with goiter size, thyroid hormone level, or proptosis. However, there was a tendency for anti-thyroid therapy to require a shorter time for FT4 normalization in blocking TBII subjects, suggesting that blocking-type anti-TSHR antibody plays some role in the pathophysiology of Graves' disease. In conclusion, blocking type anti-TSHR antibodies are often found in the sera of Graves' patients when the blocking-specific radioreceptor assay is applied.