The 72 kD heat shock protein (hsp 72) has been postulated to play a role in the development of autoimmune disease and has been shown to be overexpressed in the Graves' disease thyroid gland. The expression and modulation of the 72 kD heat shock protein were therefore studied in cultured human thyroid epithelial cells (TEC). TEC from normal thyroid tissue as well as from non-toxic goitres, thyroid adenomas and Graves' disease thyroids were analysed by Western blotting. Potential modulatory effects of heat shock treatment, TSH, IFN gamma, different serum supplements, sodium iodide and sodium selenite were investigated. Hsp 72 was detectable in cells from all tissue types under basal culture conditions and, at increased concentrations, after heat shock treatment. Quantitative evaluation of Western blotting results by densitometer scanning revealed that hsp 72 concentrations were identical in TEC from normal and diseased thyroids. IFN gamma increased the expression of hsp 72 under basal culture conditions as well as after heat shock treatment. TSH had no effect. Sodium iodide did not affect hsp 72 under basal culture conditions, but augmented the susceptibility of TEC to the effect of heat shock treatment. In contrast, sodium selenite had no effect on the expression of hsp 72. These results demonstrate that local as well as environmental factors may facilitate the induction of hsp 72 in TEC. This may be an important factor for the initiation and progression of thyroid autoimmunity. The stimulatory effect of iodide on hsp induction may also provide an explanation for the frequent occurrence of thyroid autoimmune diseases after iodine exposure.