Paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) administered by a 3-hour infusion causes a rapid decline to and recovery from the hematologic white blood cell nadir. This suggests that biweekly administration of paclitaxel alone or in combination with drugs that have limited hematologic toxicity may be possible. The first study discussed in this report tried to determine the tolerability and activity of biweekly paclitaxel administered in combination with cisplatin in patients with metastatic breast cancer. Subsequently, after an impressive response rate, a second study of biweekly paclitaxel alone was initiated to attempt to discern which activity spectrum and which toxicities were due to paclitaxel and which were due to cisplatin. Patients with metastatic breast cancer who received up to one prior adjuvant chemotherapy regimen were eligible for both studies. Paclitaxel was administered intravenously by a 3-hour infusion followed by intravenous cisplatin biweekly in the ambulatory setting. In the second study, paclitaxel was administered alone. Twenty-nine patients have been entered in the combination study, of whom 27 had received prior adjuvant therapy and 23 had received prior anthracyclines. Dose-limiting toxicity for the phase I study of paclitaxel and cisplatin proved to be a failure to recover neutrophil counts greater than 750 cells/microL by day 14. The maximum tolerated dose was paclitaxel 90 mg/m2 and cisplatin 60 mg/m2 every 2 weeks. Nonhematologic toxicities were mild and included fatigue, arthralgias, and nausea and vomiting. At this time, the 27 patients evaluable for response have achieved an 85% response rate and an 11% complete response rate. Complete responses have been seen in soft tissue, lung, and nodal disease. All patients with complete responses have had previous anthracyclines. The biweekly paclitaxel-alone study is still accruing patients. The current paclitaxel dose level is 150 mg/m2. It is still too early to evaluate response; however, response rates appear to be less impressive than those seen with combined paclitaxel/cisplatin. The final results of these studies are pending.