Taxol is a unique plant product that promotes in vitro assembly of microtubules. In a phase I trial, adults with advanced solid tumors were given taxol (formulated with cremophor EL and dehydrated alcohol) as a 3-hour iv infusion every 21 days. The total dose administered ranged from 15 to 230 mg/m2 in nine escalation steps. Leukopenia, thrombocytopenia, nausea and vomiting, alopecia, stomatitis, transient rashes, increases in serum triglyceride levels, and hypersensitivity reactions were observed. Hypersensitivity reactions characterized by acute dyspnea, generalized erythema, and hypotension immediately following the initiation of the taxol infusion occurred in three of five patients receiving greater than or equal to 190 mg/m2 (18% of patients overall). No antitumor activity was observed. Hypersensitivity reactions constituted a treatment-limiting toxicity for this preparation of taxol given on this schedule over the dosage range examined. With the severity and unpredictability of the hypersensitivity reactions, further usage of taxol is not indicated with this drug formulation on this administration schedule. Further studies are warranted to uncover ways to permit the safe administration of taxol.