In vitro detection of T-cell responses to autoantigens in type 1 diabetes is recognized as being technically challenging. We aimed to accurately measure cellular responses to proinsulin in patients with diabetes, and speculated that presentation of antigen by dendritic cells (DCs) would enhance the sensitivity of the peripheral blood assay. Antigen was mannosylated to facilitate uptake through DC surface mannose receptors to further improve the assay. Whole proinsulin, as well as mannosylated peptides of proinsulin, were combined with peripheral T cells and autologous immature DCs in a proliferative assay in a panel of newly diagnosed type 1 diabetic patients. The DC-based assay detected responses to proinsulin in five of 15 diabetic patients compared to one of 15 diabetic patients detected using the standard mononuclear cell assay. When the results of all patients were combined, the DC assay, but not the mononuclear cell assay, had a proinsulin response that was significantly higher than background (P < 0.001). The DC assay was, however, associated with high autologous mixed lymphocyte reactions that possibly masked responses in individual patients. Mannosylated antigen was taken up in larger quantities than non-mannosylated antigen, but not presented any more powerfully. Our data suggest that autologous DC-based assays are more powerful than standard peripheral blood mononuclear cell assays. However, they are compromised by high autologous mixed lymphocyte reactions and this requires addressing before they can be used as a routine readout of in vitro peripheral T-cell responses.