We studied the distribution of binding sites for anti-peroxidase monoclonal antibody and anti-microsomal antibodies on isolated human thyroid follicles and a human thyroid cell line. Both open follicles and cells were incubated first with antibodies at +4 degrees C, then with colloidal gold labelled protein A. The topography of the binding sites for monoclonal anti-peroxidase antibody corresponded closely to the expected cell surface distribution of endogenous thyroid peroxidase since labelling was observed at the apical cell surface of the follicles. Furthermore, labelling was restricted to the microvilli level; while smooth membrane territories were devoid of binding sites. In some cases, incubations at 4 degrees C were followed by warming the follicles and cells up to 37 degrees C for 20 minutes in order to study internalization of ligands. Ligands were then observed in intracellular organelles: endosomes and lysosomes. Essentially the same results were observed when human antibodies to the microsomal antigen were used. Controls with microsomal antibodies depleted in anti-peroxidase were negative. In conclusion these findings show that: 1) thyroid peroxidase is present in limited areas on the apical cell surface, 2) labelling of follicles and cells by the anti-microsomal antibodies had the same pattern of distribution as the monoclonal anti-peroxidase antibody, thus suggesting that they recognize the same apical antigens, and 3) TPO/MIC antigen traffics from the cell surface towards lysosomes when the cells are incubated at 37 degrees C.