Expression of antigenic proteins in the periphery may result in immunologic tolerance, immunologic ignorance, or autoimmunity. It is not clear why some Ags induce tolerance, whereas others activate Ag-reactive T cells. The physical nature of the Ag, the developmental timing of Ag expression, the priming of reactive lymphocytes, and the level of Ag expression are possible factors determining the immunologic response to extra-thymic Ags. Expression of the whole SV40 large T Ag (SV40-T) induces transformation of T antigen-expressing cells in vivo, and this phenomenon has been postulated to be the triggering event that leads to autoimmunity in some transgenic mouse models. Here we present a model in which a nononcogenic, yet antigenic, fragment of the SV40-T (SV40-Tfrag) is expressed specifically in pancreatic islet beta-cells. In contrast to whole SV40-T transgenic mice, SV40-Tfrag mice that are also transgenic for a TCR specific for the SV40-T are ignorant of Ag in vivo. They do not respond in vivo to the tissue-specific SV40-Tfrag Ag even after priming, but are fully responsive in vitro. This immunologic ignorance cannot be broken after activated SV40-T reactive T cells are transferred into sublethally irradiated mice expressing the islet-specific SV40-Tfrag. However, similar adoptive transfer experiments in mice co-expressing B7-1 and SV40-Tfrag on islet cells specifically lead to Ag recognition and diabetes. This demonstrates that in some circumstances the presence of (primed) reactive T cells is not sufficient to break tolerance; rather, costimulation is additionally required to elicit an autoimmune response. This also suggests that SV40-T-induced cellular transformation is important for the autoimmune response directed against SV40-T in other tissue-specific transgenic models.