Epithelial expression of class II antigens encoded by the major histocompatibility complex (MHC) has been proposed as a means by which autoimmune thyroid disease may be initiated and maintained. We studied a rat thyroid epithelial cell line (FRTL-5), which constitutively expresses class I (OX18) but not class II (OX6 or OX17) determinants to quantify in vitro MHC antigen induction using flow cytometry. Recombinant rat gamma interferon (rIFN-gamma) induced dose-dependent expression of OX6 (I-A) antigen at greater than 48 h (maximum 80-90% of cells in culture at 100 U/ml), which was abrogated by DB-1, a monoclonal antibody to rat IFN-gamma. OX17 antigen (I-E) was also induced (86%) and OX18 (class I) markedly increased under these conditions. Other thyroid-active agents including the calcium ionophore A23187, dibutyryl cyclic AMP, thyroid-stimulating autoantibodies from Graves' disease patients (LATS), and TSH, caused no I-A induction. Supernatants from spleen cells stimulated with plant lectins (concanavalin A or phytohaemagglutinin), but not lectin alone, evoked substantial class II induction, which was inhibited by DB-1. These findings suggest that IFN-gamma is the central mediator of thyroid epithelial class II expression. FRTL-5 provides a powerful model for the analysis of thyroid MHC class II dynamics and a potential means of analysing the role of epithelial class II in autoimmune pathogenesis.