Maytansine, an ansa macrolide, was evaluated in an early clinical trial in 40 adult patients with various solid tumors. Severe nausea and vomiting, sometimes associated with watery diarrhea and abdominal cramps, and liver function abnormalities, mainly elevation of serum glutamic--oxaloacetic transaminase levels, together constituted what we considered dose-limiting toxicity. Mild hematologic toxicity (mainly thrombocytopenia), neurotoxicity, and possibly cardiac toxicity were also noted. No antitumor effect was seen. An iv dose of 0.750 mg/m2 on days 1, 3, and 5 (total dose, 2.25 mg/m2) repeated every 4 weeks is recommended for Phase II trials.