Maytansine has significant antitumor activity in animal model systems. The initial clinical trial of maytansine was carried out in 38 adult solid tumor patients. Five daily bolus injections were repeated at 21-day intervals. A total of 78 courses were administered over a dose range of 0.1--0.8 mg/m2/day X 5 days. Gastrointestinal toxicity was dose-related and dose-limiting at doses of greater than or equal to 0.5 mg/m2. Dose-related neurotoxicity was also observed. No drug-related myelosuppression or change in serum creatinine level was seen. Hepatic toxicity was subclinical and reversible. Of 16 patients evaluable for response, two with breast cancer had therapeutic benefit. Phase II studies of maytansine are recommended at a starting dose of 2.0--2.5 mg/m2/course repeated at 21-day intervals.