Passive immunotherapy with monoclonal antibodies is a routinely performed but cost intensive treatment against certain cancers. Induction of humoral anti-tumor responses by active peptide immunization has therefore become a favorable treatment concept. We have recently identified three peptides representing B-cell epitopes of the extracellular domain of Her-2/neu each of them inducing Her-2/neu specific immune responses with anti-tumor activity in vitro. The present study was performed to evaluate the in vivo protective capacity of a combined vaccination with these three peptides in FVB/N transgenic mice spontaneously developing c-neu overexpressing breast cancers. The three Her-2/neu peptides coupled to tetanus toxoid were administered with or without addition of recombinant IL-12. At the time all untreated mice had developed tumors about 40% of peptide-immunized mice and nearly 60% of mice immunized with the peptide vaccine co-applied with IL-12 remained tumor free. Moreover, co-administration of IL-12 had a significant impact on the retardation of tumor progression. The enhanced anti-tumor efficacy of the vaccine by IL-12 was associated with a Th1 biased immune response as demonstrated by an increased IFN-gamma production in vitro and elevated Her-2-specific IgG levels. Our findings clearly demonstrate that this multi-peptide vaccine is effective in tumor prevention and support its use against minimal disease, drug-resistant tumors or even for prophylaxis against cancers overexpressing Her-2/neu.