A vaccine containing a HER2/Neu-derived epitope (amino acids 654-662) (GP2), and combined with granulocyte-macrophage colony-stimulating factor (GM-CSF), with potential antineoplastic and immunoadjuvant activity. Upon vaccination, GP2 may activate the immune system to mount a cytotoxic T-lymphocyte (CTL) immune response against HER2/Neu expressing cancer cells. GM-CSF may potentiate a tumor-specific cytotoxic T-lymphocyte (CTL) response against cancer cells expressing the HER2/Neu antigen. HER2/neu, a tumor associated antigen (TAA), is overexpressed in a variety of tumor cell types and is highly immunogenic.