The carcinoembryonic antigen (CEA)-derived peptide CAP1 and heteroclitic peptide CAP1-6D are stimulators of HLA-A*A0201 restricted CEA-specific T cells in vivo and in vitro. The goal of this study was to evaluate differences in T cell responses to peptide and modified peptide antigens from CEA. The heterogeneity of responses among individuals is potentially important for the design of future CEA-directed immunotherapy trials. Peripheral blood mononuclear cells from blood donors were stimulated with peptide, IL-2, and IL-7. Weekly, microcultures were restimulated with irradiated, autologous peptide-loaded peripheral blood mononuclear cells and expanded in IL-2. Established T cell lines were tested by cytokine release assays using peptide-loaded T2 targets. T cell avidity was measured by cytokine release using targets expressing diminishing concentrations of peptide. Fine specificities were measured using targets loaded with alanine-substituted CAP1 peptide. Tumor recognition was measured using HLA-A*A0201/CAP1-transduced COS tumor targets. Varied responses to CAP1 and CAP1-6D were seen among individuals. The immunogenicity of CAP1 or CAP1-6D was donor dependent. Many T cells recognized one peptide but did not cross-recognize the altered peptide. The avidities of T cell lines were moderate to low, and fine specificities were consistent with a narrow antigen-specific repertoire. CAP1-6D-based immune therapy may not be optimal in some patients with CAP1-specific precursors. The T cell repertoire may be a central contributor to the limited responses seen with CEA-directed immunotherapy to date. Treatment strategies designed to alter or expand the T cell repertoire against CEA should be considered for trials.