Strong association between diabetes and displacement of mouse anti-rat insulinoma cell monoclonal antibody by human serum in vitro. In an attempt to identify novel pancreatic beta-cell surface antigens, mouse monoclonal antibodies (MoAbs) were raised against rat insulinoma (RIN5F) cells with standard techniques. Several clones were identified whose antibodies bound specifically to RIN5F cells but not to other rat, mouse, and human target cells. Each of these MoAbs was radiolabeled, and the specificity of binding of each MoAb was determined by the ability of excess cold homologous MoAb to displace the labeled MoAb. Six RIN5F cell-specific MoAbs of different epitopic specificities were identified. The relevance of these beta-cell epitopes to human insulin-dependent diabetes (IDDM) was demonstrated by the differential ability of human serums from control and diabetic children to displace the radiolabeled MoAbs from the RIN5F cells. Serums from 333 children without diabetes or a family history of diabetes and from 156 newly diagnosed IDDM patients were tested. Only one IgM MoAb was specifically displaced by the IDDM serums, i.e., 146 of 156, compared to serums from control children, i.e., 10 of 333. With immunofluorescence, the serum component responsible for the displacement of the mouse MoAb was identified as IgG. Most of the positive control serums were from children with active autoimmune thyroiditis. Serums from children with other forms of glucose intolerance did not displace MoAb 1A2. There was no correlation between age and the degree of displacement of 1A2. Thus, the displacement of 1A2 is a specific and sensitive marker of diabetes susceptibility easily applicable to mass screening. 