The identification, quantification, and characterization of T-cells reactive with the islet autoantigens GAD65, proinsulin (PI), and tyrosine phosphatase–like molecules IA-2 and phogrin are major research goals in type 1 diabetes. In the Immunology of Diabetes Society First Workshop on Autoreactive T-Cells, the quality of recombinant preparations of these autoantigens was identified as a significant weakness, a finding that may account for much of the inconsistency in published studies of peripheral blood T-cell reactivity to islet autoantigens. Poor antigen quality has also hampered the development of novel technologies for the detection of islet-reactive T-cells. For these reasons, in the present study, several preparations of GAD65, PI, and IA-2 were collected and evaluated for endotoxin content, ability to stimulate a panel of relevant T-cell clones, and inhibitory effects on proliferation to unrelated third-party antigens. Through this process, we have been able to identify preparations of GAD65 and IA-2, generated in insect cells using the baculovirus expression system, that stimulate relevant clones and display low inhibitory effects on third-party antigens. In addition, we characterized a PI preparation generated in bacteria as being free of effects on proliferation to third-party antigens and low in endotoxin content. These preparations are important to promote the development of robust and sensitive assays of islet-reactive T-cells in patients with type 1 diabetes or patients at high risk for developing the disease.