The Southwest Oncology Group studied the response rate and toxicity of mitoxantrone (7.5 or 10 mg/m2 to 12.0 mg/m2) and cis-platinum (100 mg/m2) in 30 patients with advanced breast cancer as second-line therapy. There were 2 partial responses in 29 eligible patients. Toxicity was considerable, with 27 patients having grade 3 or 4 toxicity. Grade 3-4 toxicity included vomiting, thrombocytopenia, granulocytopenia, leukopenia and anemia. The combination of mitoxantrone plus cis-platinum has minimal activity as second-line therapy in metastatic breast cancer.