Based on their single-agent activity in metastatic breast cancer, paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) and doxorubicin have been alternated and combined in Eastern Cooperative Oncology Group studies to identify a tolerable dose and schedule. A pilot trial in patients who had received no more than one prior chemotherapy regimen alternated paclitaxel 200 mg/m2 and doxorubicin 75 mg/m2 every 3 weeks. Seven of 12 patients had objective (complete plus partial) responses. A second (phase I) trial combined doxorubicin as an intravenous push and paclitaxel as a 24-hour continuous infusion. To evaluate the effect of the schedule on toxicity, the drug administration sequence (doxorubicin-->paclitaxel or paclitaxel-->doxorubicin) was alternated both between and within patients. initial doses were paclitaxel 150 mg/m2 and doxorubicin 50 mg/m2 in six patients, with a subsequent six patients receiving 175 and 60 mg/m2, respectively. Dose-limiting mucositis was seen at the second dose level when paclitaxel preceded doxorubicin, suggesting an important role for the administration sequence in determining toxicity (and possibly efficacy) in this regimen. These results support the sequence of doxorubicin 50 mg/m2 followed by paclitaxel 150 mg/m2 as the maximum tolerated dose. This combination has been incorporated as a treatment arm in an ongoing randomized prospective phase III Intergroup trial. By comparing the combination with both drugs as single agents (with crossover to the opposite agent at disease progression), investigators will attempt to assess response, toxicity, and time to progression as well as the degree of cross-resistance between the two agents. Given the poor prognosis of patients with advanced breast cancer, the three arms also will be evaluated in terms of patients' quality of life.