We studied the biochemical properties and the reactivity of 13 monoclonal anti-thyroglobulin antibodies, obtained by fusing P3 × 63 (8653) myeloma cells with spleen cells from CBA and C57BL 6 mice previously immunized with syngeneic thyroglobulin. We demonstrated that among the 13 anti-thyroglobulin monoclonal antibodies that we studied, each of them was specific for only syngeneic thyroglobulin, as each of them was able to cross-react with heterologous or xenogeneic thyroglobulin. Two major kinds of monoclonal antibodies were obtained: the first ones are able to block in vitro primary syngeneic sensitization and, moreover, they mutually compete for thyroglobulin binding; the second ones are not involved in this in vitro PSS and do not compete for binding to thyroglobulin. These results suggest that a defined particular epitope, borne by thyroglobulin and expressed on the thyroid epithelial cells, is responsible for in vitro primary syngeneic sensitization.