Chlorozotocin was given to 37 patients with advanced malignant tumors in a daily X 5 schedule at 6-week intervals. Total iv doses for each course ranged from 75 to 200 mg/m2. Myelosuppression was dose-limiting, with a platelet count depression regularly observed at doses of greater or equal to 150 mg/m2; leukopenia occurred only at the highest dose level. Nausea and vomiting were mild and uncommon. No hyperglycemia or adverse drug-related effects on renal or hepatic function were observed. No major antitumor activity occurred; however, three patients with renal cell carcinoma and one patient each with lung cancer, ovarian carcinoma, and Hodgkin's disease had minor objective decreases in tumor size. A dose range of 150--200 mg/m2 iv for each 5-day course is recommended for phase II studies.