To study the role of interferon-gamma (IFN gamma), which is released from activated lymphocytes in the development of lymphocytic thyroiditis, we investigated the effects of IFN gamma on the morphology of mouse thyrocytes, using scanning electron microscopy. Thyrocytes were cultured in monolayers for 5-7 days and incubated with TSH (10 mU/ml) for 1 h before fixation. IFN gamma (200 U/ml) was added to the culture medium for 1 h to 5 days before TSH stimulation. Coculture of thyrocytes with IFN gamma for more than 24 h inhibited TSH-induced morphological changes in the thyrocytes; IFN gamma inhibited the increase in the number of and elongation of microvilli and the appearance of pseudopods. IFN gamma also reduced the number of actin filaments in thyrocytes, as confirmed by immunofluorescence photometry. These results suggest that IFN gamma inhibits the morphological response of thyrocytes to TSH stimulation by the reduction of actin filaments.