Effects of the addition of MER, a nonspecific, nonviable immunostimulant, to two combination chemotherapy programs were explored in patients with metastatic breast cancer. Patients were randomized to either CDVFP [cyclophosphamide (C), doxorubicin (D), vincristine (V), fluorouracil (F) and prednisone (P)] or CD alternating with methotrexate (M) and F (CD/MF). Each group was also randomized to receive MER, 0.4 mg S.C. every four weeks or no immunotherapy. The response rates were CDVFP 56%, CDVFP + MER 54%, CD/MF 43%, and CD/MF + MER 43%. No significant differences were noted in response rate. Median durations of response and survival were similar for each group: CDVFP 16.2 and 25.2 months, respectively; CDVFP + MER 14.0 and 23.3 months, CD/MF 12.1 and 26.1 months, and CD/MF + MER 15.5 and 25.6 months. Patients who achieved CR frequently had soft-tissue disease (7/17) and patients with disease in 1 or 2 metastatic sites had a significantly higher response rate than those in greater than or equal to 3 sites. MER did not enhance response rate, duration of response, or survival. Also MER did not diminish myelosuppression.