The Phase I clinical trial of the combination of 120-hour continuous intravenous infusion of vinblastine with oral PSC 833 has shown activity in patients with advanced malignancies, particularly renal cell cancer. The MTD of vinblastine in combination with the oral drink solution of PSC 833 was determined to be 0.9 mg/m2/day for five days and 12.5 mg/kg po q 12 hours for eight days, respectively. For the soft gel capsule formulation, the MTD was determined to be 0.6 mg/m2/day vinblastine for five days and 4 mg/kg po q 6 hours PSC 833 for eight days. Ataxia was the dose limiting toxicity. Of the 46 patients, two complete remissions and one partial remission were seen among 29 patients with renal cell carcinoma.