Recombinant human ErbB3 fragment (rhErbB3-f) immunization. rhErbB3-f is a recombinant human ErbB3 receptor fragment produced by expression in E. coli. After purification and formulation with adjuvant, it can be used for immunization to induce the production of specific anti-ErbB3 antibodies for the treatment of patients with breast cancer characterized by the overexpression of ErbB2. rhErbB3-f, therefore, is as a therapeutic cancer vaccine.