Twenty-four patients with metastatic breast cancer, all but 1 pretreated with one or more chemotherapeutic regimens, were entered in a pilot study to assess toxicity and efficacy of the combination mitoxantrone and vinblastine. Dominant sites of metastases were viscera in 9 patients, bone in 10 and soft tissues in 5. All patients received mitoxantrone 10 mg/m2 i.v. on day 1 and vinblastine 1.7 mg/m2 by 4 hour infusion on days 3, 4 and 5, every 3-4 weeks. Objective responses (1 CR, 7 PR) were observed in 8 (38%) of the 21 evaluable patients. Median duration of response was 10.5 months. Of 12 patients pretreated with an anthracycline containing regimen, 4 responded (1 CR and 3 PR). Major toxicity was myelosuppression, grade 4 in 2 cases and grade 3 in 2 others. No evident alopecia was observed and only 1 case of grade 1 cardiac toxicity. In conclusion, mitoxantrone followed by vinblastine is an effective regimen in metastatic breast cancer also in pretreated patients, and previous anthracycline administration does not seem to reduce the percentage of response. Moreover, toxicity is generally mild.