A radioreceptor assay was used to measure thyroid stimulating antibodies (TSAb) during the acute and recovery phases in 7 patients with subacute thyroiditis. High levels of TSAb were detected in 4 patients during the acute phase. In two other patients, tests were borderline positive. In the latter two patients tests were negative by two weeks, whilst in the 4 patients with strongly positive tests initially, levels persisted for several weeks, falling to within the normal range by 3 months in the two patients in whom repeated tests were carried out. TSAb probably do not play a role in the transient hyperthyroidism commonly seen in this disorder, since the detection of TSAb did not correlate with clinical or biochemical evidence of hyperthyroidism. Thus, in this disorder, TSAb are apparently able to bind to thyroid membranes in vitro, but not stimulate the gland in vivo. On the other hand TSAb could possibly play a role in the recovery phase. The possible role of suppressor cells in the development of the transient immune abnormalities which are associated with temporary thyroid damage is discussed.