Type I, insulin-dependent diabetes (IDD) results from an autoimmune response against the insulin producing pancreatic beta cells. This autoimmune reaction involves both humoral and cell-mediated factors; nevertheless, the relative role of each remains unresolved. Furthermore, while adoptive transfer experiments have provided evidence for the role of T cells in beta cell destruction, the specific events which initiate leukocyte migration into the islets (insulitis) are unknown. Earlier studies indicated that NOD pancreatic beta cells may bind small amounts of autoantibody. Because of the possible importance of an early humoral response to the initiation of insulitis and subsequent disease, we have investigated a number of aspects of this phenomenon to determine the nature and specificity of the early autoantibodies as well as the time at which autoantibody binds to beta cells. Results of this study demonstrate that NOD/Uf mice are sensitized to islet-cell associated antigens, including GAD, prior to the first appearance of insulitis; that a small percentage of the beta cells of NOD/Uf mice have autoantibody bound to their surface prior to insulitis; that sera collected from preinsulitis NOD/Uf mice contain autoantibodies which will bind to beta cells of both IDD-prone and IDD-resistant mice; and that the autoantibodies which bind pancreatic beta cells are predominantly IgM with lesser amounts of IgG and IgA. These findings suggest that, in the natural course of IDD, insulitis may develop in response to an initial autoantibody-mediated injury of beta cells.