B7-1 is a co-stimulatory molecule that signals T-cells that recognize antigen to proliferate and differentiate into effector T-cells. The same cell must present antigen and express co-stimulatory molecules, such as B7-1, to activate naive T-cells. Thus, tissues that do not express co-stimulatory molecules would not be expected to induce immune responses, while expression of a co-stimulator on tissue cells may convert them into effective antigen-presenting cells and induce autoimmunity. To test this, transgenic mice have been generated that express B7-1 on the beta-cells of the pancreatic islets of Langerhans. On a B6 genetic background, B7-1 expression on beta-cells does not predispose to diabetes. B6 mice are resistant to diabetes. However, when B7-1 is expressed on the beta-cells of B6 mice backcrossed once to the genetically susceptible NOD strain, the onset of diabetes is accelerated and the autoimmune attack intensified. This illustrates that B7-1 is a very potent co-stimulatory molecule in vivo and that its presence on the surface of tissue cells can potentiate the autoimmune process.