Strains of rat differ in their susceptibility to experimental autoimmune thyroiditis (EAT). We recently observed that the normal Wistar rat has lymph node (LN) T cells which recognize the newly available cloned Wistar thyroid cell line (WRT) and/or rat thyroglobulin (rTg). We have now cloned thyroid-specific T cells and characterized their interaction with the WRT target cell. Twenty-three T cell clones were tested for their reactivity to syngeneic thymocytes, WRT cells alone, or WRT cells with thymocytes. All the clones were of the CD4+ CD8- phenotype. Seven of 23 T cell clones proliferated in the presense of WRT cells alone or with the combination of WRT cells and thymocytes, exhibiting stimulation indices of 1.5 to 5. In all but one of the T cell clones responding to WRT cells alone was there no evidence that the additional presence of thymocytes supplied a stronger "second" proliferative signal than the WRT cells. These WRT-reactive clones which were able to be more extensively characterized were MHC class II restricted, secreted rat interferon (IFN)-γ in response to WRT cell exposure, and one clone showed cross-reactivity with rTg antigen. Induction of WRT cell MHC class II antigen by prior treatment with IFN-γ failed to further enhance the WRT cell-induced T cell proliferation. These data provide the first evidence for direct antigen presentation by thyroid epithelial cells (TECs) in the absence of other antigen-presenting cells. Further-more, they provide evidence that TECs are able to provide the appropriate "second" signals required for T cell activation and successful autoantigen presentation.