Summary: Thirty HLA-A2+ women with metastatic breast cancer received up to 14 vaccinations with MDA-MB-231-CD80, an HLA-A2 + allogeneic breast cancer cell line, which had been lipofected with the cDNA for the CD80 costimulatory molecule. Tumor cells were administered with BCG or GM-CSF as an adjuvant. Sera obtained before and after vaccination were analyzed for antibodies to tumor cell lysate, MUC1, HER2/neu and p53. Since the cell line was grown in fetal bovine serum (FBS), sera were also analyzed for antibodies to FBS. Eighteen of 24 patients for whom sera were available exhibited anti-FBS activity at baseline. Eleven of these 18 patients and all six patients without baseline anti-FBS activity showed an increased titer after vaccination. The anti-FBS activity required that serum samples be absorbed in excess FBS to detect specific antibodies to tumor cell lysate. A two-fold increase in the titer of IgG specific to tumor cell lysate was observed in 6 patients. Eight of 24 patients made an antibody response to HER-2/neu, four of 24 to MUC1 and one of 24 to p53. Although antibody production to a variety of tumor cell-associated antigens was detected our results suggest that a whole cell vaccine comprising a CD80-transfected allogeneic breast cancer cell line with adjuvant BCG or GM-CSF was not a reliable method to induce significant antibody responses in women with advanced breast cancer. Key Words: Antibodies-BS-ER-2/neu-MB-231-UC1.The potential value of a vaccine that induces the production of high titer antibodies to tumor-associated antigens is illustrated by the successful treatment of patients with breast cancer and non-Hodgkins lymphoma (NHL) with exogenously administered monoclonal antibodies. Women with metastatic breast cancer that overexpresses Her2/neu experience increased survival after treatment with trastuzumab (Herceptin; Genentech Inc., South San Francisco, CA, U.S.A.), a humanized monoclonal antibody with high affinity for HER-2/neu (1,2), when given with chemotherapy (3). The FDA has also approved anti-CD20, rituximab, for the treatment of patients with CD20 + NHL (4). Furthermore, women with breast cancer who develop a natural immune response to MUC1 (5) and CEA (6) survive longer than women who fail to develop an antibody response. Production of tuSupported in part by NCI RO3 CA70299-02 (awarded to W. J. Urba) and grants from the Chiles Foundation, the Collins Medical Trust, the M.J. Murdock Charitable Trust, and a generous gift from the Flora Family Foundation.