OBJECTIVES: I. Evaluate the response rate and time to treatment failure of paclitaxel with and without the P-glycoprotein (Pgp) antagonist PSC 833 in advanced breast cancer. II. For each treatment arm, relate paclitaxel AUC (area under curve), and/or time above .05 um/L, to myelosuppression and/or response. III. To obtain preliminary estimates of MDR in this group of patients by measuring MDR1-Pgp immunostaining in pretreatment biopsies in 20 patients and biopsies taken at the time of progression.