OUTLINE: The study design is a prospective, double-blinded, randomized study. Patients who have completed first-line chemotherapy for metastatic breast cancer and have either nonprogressive disease or no evidence of disease following completion of first-line chemotherapy (includes bone marrow transplants and stem cell rescue) will be randomized to either the THERATOPE vaccine or the control vaccine. Patients are stratified at entry according to disease status (i.e., either no evidence of disease or nonprogressive disease) and whether or not they are receiving hormonal therapy for metastatic disease while on study. Arm I: Patients receive intravenous cyclophosphamide on day -3, followed by 4 subcutaneous vaccinations with THERATOPE STn-KLH vaccine combined with Detox-B Stable Emulsion at 0, 2, 5, and 9 weeks. Arm II: Patients receive the control treatment of intravenous cyclophosphamide on day -3, followed by 4 subcutaneous vaccinations with keyhole limpet hemocyanin (KLH) vaccine combined with Detox-B Stable Emulsion at 0, 2, 5, and 9 weeks. Patients with stable or responding disease may receive the THERATOPE STn-KLH vaccine or control without Detox-B Stable Emulsion at weeks 13, 17, 21, and 25. Patients without unacceptable toxic effects or disease progression may continue on maintenance therapy at 3 month intervals.