Autoimmune responses are initiated by MHC class II-restricted T cell responses directed against tissue-specific autoantigens. Furthermore, HLA-DR expression in thyroid epithelial cells is a prominent feature of autoimmune thyroid disease. In the present work, we were particularly interested in a phenothiazine, a neuroleptic and anti-depressant drug of pharmacologic importance named alimemazine. Our interest in this compound stems from previous findings of immune effects of this and other phenothiazines. We demonstrate that MHC class II Ags can be experimentally induced on thyroid cells by pharmacologic concentrations of alimemazine, a drug commonly used in psychiatry. In contrast, MHC class II Ags were not induced on the lymphoid cell lines Raji and Jurkat. Expression of MHC class II Ag on the surface of the cloned human thyroid cell hybridoma, GEJ, was demonstrated by flow cytometry. Moreover, by using Northern blot and Southern blot analyses, this finding was confirmed at the molecular level in GEJ and in murine thyroid epithelial cell cultures, respectively. The functional role of phenothiazine-, de novo-induced MHC class II Ags on thyroid cells was assessed by both syngeneic murine thyroglobulin-specific and allogeneic proliferative T cell responses. These results suggest that antidepressant drugs of the phenothiazine type could play a role in the induction and the perpetuation of thyroid autoimmune disorders, through induction of class II restriction elements on normally class II-negative thyroid epithelial cells.