Mitomycin C, 20 mg/m2 day 1, and vinblastine, 0.15 mg/m2 days 1 and 21, were administered intravenously to 31 patients with refractory previously treated advanced breast cancer. Courses were repeated every six to eight weeks when adequate hematologic recovery permitted. Thirty of the 31 patients were evaluable; one had a complete response and 11 had a partial response. Four patients had stabilization of their disease. There was a 12/30 (40%) response rate with a median response duration of 127 days (range 69--412 days). Toxicity included moderate to severe myelosuppression, infections, hemorrhage, neurologic symptoms, pulmonary toxicity and mucositis. In previously treated patients with poor prognoses, the combination of mitomycin C and vinblastine appears to be a worthwhile secondary treatment for metastatic breast cancer.