The oncogene HER-2/neu is genetically amplified and overexpressed in a large number of human adenocarcinomas and has been implicated in the tumorigenic phenotype. Although it is a nonmutated self-protein, it is barely detectable in adult tissues, and immune responses toward it have been described in a number of patients. It is, thus, an attractive candidate antigen for the immunotherapy of cancer patients. HLA-A2+ patients with metastatic breast, ovarian, or colorectal adenocarcinomas that overexpressed HER-2/neu were immunized with the HLA-A2-binding epitope p369-377 (p369). Patients were treated by repeated immunization with 1 mg of p369 in Freund's incomplete adjuvant every 3 weeks. Peripheral blood mononuclear cells were collected prior to immunization and following two and four immunizations and were stimulated in vitro with peptide and assayed for peptide and tumor recognition. In three of four patients, peptide-specific CTLs were detected in post- but not preimmunization blood. These CTLs recognized peptide-pulsed target cells at peptide concentrations of > or =1 ng/ml yet failed to react with a panel of HLA-A2+ HER-2/neu+ tumor lines. In addition, infecting HLA-A2+ cells with recombinant vaccinia virus encoding HER-2/neu or up-regulating HLA-A2 with IFN-gamma in HER-2/neu+ cells also failed to confer reactivity by p369-reactive T-cells. A T-cell response to the HLA-A2 binding epitope p369 can be easily generated by immunizing patients with peptide in Freund's incomplete adjuvant. However, the CTLs failed to react with HER-2/neu+ tumor cells. Further studies are needed to determine whether and how HER-2 might serve as an antigen for tumor immunotherapy.