The value of immunotherapy as an adjuvant to chemotherapy for advanced breast cancer is an unsettled question. To clarify this issue, 71 women with measurable or evaluable metastatic breast cancer were randomized to receive cyclophosphamide, 5-fluorouracil, and prednisone (CFP) with or without methanol-extracted residue of Bacillus Calmette-Guerin (MER). The total regression rates were 52% (CFP) and 39% (CFP + MER), including complete regression rates of 13% (CFP) and 65% (CFP + MER). The median duration of regressions for CFP-treated patients was 257-261 days and for CFP + MER-treated patients was 385 days. The median time to progression was 248-261 days in the CFP group and 159 days in the CFP-MER group. Projected median survival for both treatment groups is 20 months. Immunotherapy (MER) as used in this study does not appear to augment regression rates or vurvival for patients with advanced breast cancer receiving CFP.