Maytansine, a new ansa macrolide antitumor antibiotic, was administered to 60 patients as part of a phase I study. The doses given ranged from 0.01 (starting level) to 0.9 mg/m2 for 3 days. The toxic effects encountered consisted principally of nausea, vomiting, diarrhea, and occasionally, stomatitis and alopecia. Superficial phlebitis was also encountered and occurred when the drug was diluted in a volume of less than 250 ml. Myelosuppression occurred infrequently; it was almost regularly associated with abnormal liver function tests. Antitumor activity was detected in one patient each with melanoma, breast carcinoma; and head and neck clear cell carcinoma. Further studies are indicated with this compound since it has shown evidence of activity with little or no myelosuppression.