A total of 156 patients with metastatic breast cancer were entered into a prospective multi-center trial in September 1975. All patients were treated monthly with vincristine, adriamycin and cyclophosphamide (VAC) six times, followed by 5-fluorouracil, methotrexate and cyclophosphamide (FMC) until progression was documented. By random assignment, the patients received 5 mg/m2 Corynebacterium parvum (CP) subcutaneously on day 1, in addition to VAC/FMC. Of the 150 evaluable patients, 33 of 76 (45%) and 36 of 74 (49%) had complete or partial response to VAC/FMC plus CP, respectively. The Kaplan-Maier curves of duration of remission and survival were almost identical (medians 14.5 vs 12.1 months and 22.2 vs 21.1 months, respectively). The hematologic and gastrointestinal toxicity were also similar in the two study groups. However, 19 of 74 (26%) patients developed skin ulcers after repeated injections of CP. These patients showed prolonged survival (P = 0.002, log rank test). These results suggest that adding nonspecific immunostimulation with CP to currently available chemotherapy on day 1 is of no benefit to most patients with metastatic breast cancer, but may select an "immunoreactive subgroup with increased local toxicity and survival.