The proto-oncogene HER2/neu encodes a protein tyrosine kinase (p185HER2) that is homologous to the human epidermal growth factor receptor. Amplification and/or overexpression of HER2/neu occurs in multiple human malignancies and appears to be integrally involved in progression of some breast and ovarian cancers. Because of this fact, HER2/neu is an intriguing target for specific cancer therapeutic strategies. One such strategy is active specific immunotherapy, in which the immune system is targeted at specific antigens expressed by tumor cells. We have employed a transfected cell line that secretes the extracellular domain of p185HER2 as a source of HER2-derived immunogen in a guinea pig model. The immunized animals developed a cellular immune response, as monitored by delayed-type hypersensitivity, and antisera derived from immunized animals specifically inhibited the in vitro growth of human breast tumor cells overexpressing p185HER2. These data provide support for an immunotherapeutic approach to cancers characterized by overexpression of the HER2/neu proto-oncogene.