Human thyroid cells in culture stimulated by TSH and TSI were used in order to detect thyroglobulin expression. After three days stimulation the cells were incubated with monoclonal thyroglobulin antibody and FITC-conjugated antiglobulin. Fluorescent index (the intensity of fluorescence related to hundred analysed cells) was estimated for each experimental group. The most effective stimulation of the thyroglobulin expression was detected after TSH stimulation at the concentration of 0.1 mU/ml. TSI from active Graves' patients provoked the highest expression of thyroglobulin at concentration of 1.0 mg/ml, but the fluorescence index was lower than after TSH stimulation. The thyroglobulin expression was intracellular, large, partly confluent granules were detectable mainly in the perinuclear area. Antigen expression on the surface of cultured thyroid cells could not be detected. The morphology of thyroglobulin expression as detected by immunofluorescence, was the same after TSH and TSI stimulation. It is concluded, that both stimulating factors, i.e. TSH and TSI, are involved in the thyroglobulin expression of human thyroid cells.