To evaluate the involvement of T cell immunity in the thyroiditis induced in goitrous mice by iodide administration, we analyzed the immunological changes happening in the thyroid glands and lymph nodes during goiter involution in balb/c and athymic nude mice of similar background.In both balb/c and nude mice, goiter involution was characterized by thyroid cell necrosis and inflammation. In balb/c mice, the inflammatory infiltrate was made of numerous Ia+cells. Their number was unchanged during goiter development, but was significantly increased after 2 days of involution and remained high after 8 days. CD4 + and CD8 + T cells were also observed, some of which were clearly activated since they expressed the receptor for Interleukin-2 (IL-2R). The numbers of CD4 +, CD8 + and IL-2 R + T cells were increased during goiter as compared to control mice, and they reached a maximum at day 1 of involution.In nude mice, unexpectedly, CD4 + and CD8 + T cells were also found in the thyroid. Their numbers, as well as the number of la + cells, were significantly increased at the onset of involution, but they remained systematically lower than in the corresponding groups of balb/c mice.Iodide treatment of goitrous mice also induced modifications of the lymph nodes draining the thyroid: enlargement of the paracortical T zone, presence of germinal centers in cortical follicles, and increase of the density of IL-2 R + cells. Mesenteric lymph nodes taken as controls were unchanged.Thus, three observations suggest the involvement of T cell immunity in iodine-induced thyroid inflammation: 1. Infiltration of Ia +, CD4 +, CD8 + and IL-2 R+ cells. 2. Signs of stimulation in thyroid lymph nodes, 3. Significant differences between balb/c and nude mice, in which the inflammatory reaction is weaker.