T cells specific for the human epidermal growth factor receptor 2 (HER2) with potential immunopotentiating activity. T cells directed against HER2, overexpressed on many tumor cells, are collected from HER2-expressing tumor tissue, expanded ex vivo and, then re-introduced in the patient. Re-introduction of ex vivo-expanded HER2-specific T cells may enhance the cytotoxic T cell response against tumor cells overexpressing HER2, resulting in inhibition of tumor growth.