The development of T-cell therapy for the treatment of human malignancy has been hindered, in large part, by a lack of identifiable tumor antigens. Studies to identify potential T-cell targets in humans have been difficult because of practical problems limiting the use of in vivo immunization and a lack of reproducible in vitro priming methods. Oncogenic proteins are involved in malignant transformation and maintenance of the transformed phenotype and theoretically are potential targets to T-cell therapy. HER-2/neu protein is a protooncogene product overexpressed in a variety of human malignancies and is associated with malignant transformation and aggressive disease in human breast cancer. Previous studies have shown that some patients with breast cancer have existent helper/ inducer T-cell immunity to p185^HER2/neu protein and peptides. The current study represents initial attempts to identify candidate cytotoxic T-lymphocyte (CTL) epitopes. Synthetic peptides were constructed identical to HER-2/neu protein segments with amino acid motifs similar to the published motif for HLA-A2.1-binding peptides. Four peptides were synthesized and two were shown to be avid binders to HLA-A2.1. Two of the four peptides could be shown to elicit peptide-specific CTL by primary in vitro immunization in a culture system using peripheral blood lymphocytes from a normal individual homozygous for HLA-A2. p185^HER-2/neu protooncogene protein contains immunogenic epitopes capable of generating human CD8⁺ CTL. The identification of candidate CTL epitopes will allow studies to determine whether some cancer patients have existent CTL immunity to HER-2/neu protein. The demonstrated ability to generate human peptide-specific CTL in vitro allows screening of other oncogenic proteins to identify candidate T-cell epitopes potentially useful for future immunotherapy studies.