OBJECTIVES: I. Evaluate the proportion of patients with extreme, intermediate, and low drug resistance to paclitaxel using the Extreme Drug Resistance (EDR) Assay in patients with previously treated metastatic breast cancer. II. Assess response to paclitaxel therapy in patients who have undergone a pretreatment EDR assay. III. Assess time to tumor progression during paclitaxel therapy in patients who have undergone a pretreatment EDR assay. IV. Determine prospectively the predictive value of the EDR assay relative to clinical outcome by correlating assay results with clinical tumor response and time to tumor progression during paclitaxel therapy in these patients.