In a prospectively randomized cooperative study patients with metastatic breast cancer were treated with or without Corynebacterium parvum (C.p.)5 mg/sc/m2 on day 1 in addition to CAO/CMF (Cyclophosphamid (C) 150 mg/m2/d per os X 5 d and Adriamycine (A) 50 mg/m2 i.v. d 1 and Oncovin (O) 1.0 mg/m2 i.v. d 1. 6 CAO-cycles q 28 days later were followed by monthly CMF cycles q 28 d with Cyclophosphamide (same dose), Methotrexate 30 mg/m2 i.v. 1 and 5-fluorouracil 600 mg/m2 i.v. d 1). 76 patients entered the study until January 1, 1977. The patients' characteristics (age, cytostatic pretreatment, tumor free interval, metastatic sites and mean doses of CA) were well balanced in the two groups. The lowest values of WBC and platelets on d 14 of the monthly CAO-cycles show a mean nadir of 1,630/mm2 and 122'850/mm3 respectively in the C.p.-group and of 1,890/mm2 and 141'760/mm3 in the group without C.p. GI-toxicity was seen more frequently in the C.p. group. An improvement of symptoms was observed in 88% of the C.p.-CAO/CMF treated group and in 77% of the group without C.p. Complete and good partial remissions (greater than 50% tumor size reduction) were seen in 53% of the CAO-C.p. treated patients and in 34% of the patients in the control group without C.p. Survival data presently show a trend of improvement by the addition of C.p. to the CAO-CMF-chemotherapy regimen.