We developed an assay to determine the location of immunodominant CD4+ T-cell epitopes in any protein. The method uses CD4+ T cells from community donors in conjunction with dendritic cells derived in vitro. Synthetic peptides constructed to describe the sequence of the protein of interest are cocultured with dendritic cells and CD4+ T cells, and T-cell proliferation is measured. Data are compiled over a large replicate of human donors to pinpoint immunodominant, usually promiscuous epitope regions. We have applied this technique to a known food allergen, the Brazil nut 2S storage globulin protein, and to two potential food allergens, the Cry1Ab and Cry3Aa proteins. We show epitope data for these three proteins. This assay can be used as a tool to guide the selection and qualification of future potential food transgenes.