We performed a preliminary phase II clinical trial of MX2; 3'-deamino-3'-morpholino-13-deoxy-10-hydroxycarminomycin (KRN8602) in patients with metastatic breast cancer who had failed to respond to previous chemotherapeutic regimens after clinical evidence of systemic disease. Twelve patients at a single institute received KRN8602 at a dose of 35 mg/m2 intravenously once every three weeks. All the patients were followed-up until their disease progressed. There was one complete response lasting 17 weeks and one partial response lasting eight weeks. Among the 12 patients, World Health Organization (WHO) grades 3 and 4 neutropenia were observed in five and two patients, respectively. Grade 3 anemia was observed in three patients but severe thrombocytopenia was not observed. Grade 3 nausea/vomiting was observed in eight patients. Alopecia was not observed. The results of this preliminary phase II trial suggest a need for further testing of the anti-tumor activity of KRN8602 in patients with metastatic breast cancer.