Taxol is a unique plant-derived antineoplastic agent that appears to exert its cytotoxic effect by interfering with microtubule structure and function. In this phase I trial, in which the drug was given as a brief iv infusion every 3 weeks, the dose-limiting toxicity was leukopenia, with thrombocytopenia being seen much less frequently. Sensory neuropathy was frequently seen at the highest dosage, with numbness and paresthesias appearing in a glove-and-stocking distribution. In some cases, this appeared to be a cumulative effect. Total alopecia was common. Other toxic effects observed included nausea and vomiting, mucositis, myalgias, and phlebitis. The frequent occurrence early in the study of acute cardiovascular and pulmonary toxicity suggestive of hypersensitivity reactions was decreased in frequency and severity by prolonging the infusions and premedication with corticosteroids and antihistamines. Two heavily treated patients appeared to respond to this agent, one with non-small cell lung cancer and one with ovarian cancer. Based on these data, we recommend a starting dose in phase II trials of 212 mg/m2 in patients with minimal prior therapy and 170 mg/m2 in heavily treated patients. At least initially, these trials should be carried out in institutions familiar with the use of this drug.