51 patients with stage IIIB and IV breast cancer entered a prospective phase II study of combination chemotherapy that consisted of mitoxantrone (8 mg/m2) day 1, methotrexate (25 mg/m2) day 1, vincristine (1 mg/m2) day 2 and carboplatin (250 mg/m2) day 2 (MIMOC) given in a 3-weekly schedule. None had received prior chemotherapy for metastatic disease, although 16 patients were given adjuvant chemotherapy. Objective response to treatment was seen in 29 of 48 patients analysed for response (60%) with 8 complete responses (CR). 7 out of 8 patients with stage IIIB disease responded, 2 of them completely. Responses were seen in all sites but the best results were achieved in lung metastases with 50% CR. The median duration of response was 8 months and the median time to disease progression was 12 months. The main toxicity was nausea and vomiting which was severe in 20% of the patients. Other toxicities were mild. MIMOC was administered on an out-patient basis and appears to be effective as first-line treatment in advanced breast cancer.