A phase II clinical trial of maytansine, a stathmokinetic ansa macrolide, was undertaken in 70 patients. The maximally tolerated dose was 2.0 mg/m2 repeated at 21-day intervals. Gastrointestinal and central neurologic toxicity were dose limiting. No myelosuppression was noted. Two patients demonstrated transient responses. Therapeutic results from four other clinical trials were reviewed. Although additional clinical trials may be warranted in patients with bladder and small cell carcinoma, at the dose schedule reported, maytansine does not appear to possess a broad spectrum of antitumor activity. Additional clinical trials should be limited.