The expression of major histocompatibility complex class II molecules (Ia antigen) has been analyzed by immunoperoxidase staining in thyroids of normal C3H mice, of iodine-deficient mice with a hyperplastic goiter and of mice during goiter involution induced by administration of either a high iodide dose (HID, 10μg/day) for 0.5 to 8 days or a moderate iodide dose (MID, 1μg/day) or triiodothyronine (T3, 1μg/day) for 2 days.In normal and in hyperplastic thyroids, few interstitial cells were la positive (monoclonal antibodies, mAb, M5/114, ER-TR3). Their number was unchanged when goiter involution was induced by MID or by T., but was significantly increased (p<, 0.05) after HID. It was maximal at days 1 and 2 of involution, decreased thereafter but remained higher (p<0.05) than in controls after 8 days.The la positive cells were mainly macrophages and, to a lesser extent, dendritic cells. Macrophages were identified by their heterogeneous content and their numerous lysosomes. They were stained with anti-Mac-1 (M1/70) and anti-Mac-2 (M3/38) mAb. Dendritic cells were characterized by their slender cytoplasmic processes, indented nucleus and pale cytoplasm. They were positive for NLDC-145 and MIDC-8 mAb whose specificity for dendritic cells has been demonstrated in lymphoid organs.During the whole period of involution analyzed, la antigens were not expressed on follicular cells. Since macrophages and dendritic cells are known to be involved in the pathogenesis of immune disorders, the inflammation induced by administration of HID to iodine-deficient mice could be considered as the early step of an immunological reaction.