New cellular-based reagents are needed to diagnose type I diabetes as well as to monitor the outcomes of clinical trials at early time points. Four new monoclonal antibodies (mAbs) have been shown to demonstrate reduced binding to lymphocytes from identical twins with long-term type I diabetes relative to that observed with lymphocytes from their twin partners without diabetes or from control subjects. Biochemical analysis revealed mAb 3G12EG recognized an unidentified 45-kDa protein, whereas mAb 2E8F1 and 5B6E11 did not appear to precipitate specific proteins as detected by SDS-PAGE. Electrophoresis under reducing and nonreducing conditions and peptide mapping revealed that mAb 8F410 recognizes a novel dimeric form of HLA class I molecule. Predictions from crystallography studies suggested previously this class I dimer as the optimal activation of a single CD8 T-cell. In B-cells from both normal and diabetic individuals, the class I dimer was minimally associated with beta2-microglobulin rapidly formed in the endoplasmic reticulum. These new reagents appear to be able to identify new lymphocyte surface phenotypes associated with diabetes expression in both fresh blood samples and Epstein-Barr virus-established cell lines.