CPT-11 is a new derivative of Camptothecin. Phase I clinical study of single administration with CPT-11 was carried out by a cooperative study group. Starting from 50 mg/m2 (n), dose was escalated to 350 mg/m2 (7n). Dose limiting factor was found to be a decrease in WBC counts (especially in neutrophils), and MTD was presumed to be 250 mg/m2 or more. Nadir of WBC counts was observed after about a week, and it took 2-3 weeks for recovery. The decrease in platelet number and hemoglobin content was mild. Other side effects included G-I toxicities, alopecia, etc. However, no toxic effects on the heart, kidney, lung were observed. SN-38, main metabolite of CPT-11, was observed in blood, and excreted rapidly. Anticancer effects were suggested with dose of 165 mg/m2 or more against colon cancer, gastric sarcoma, melanoma and lung cancer. It is suggested that the optimal dose schedule for an early Phase II study is 200 mg/m2 every 3-4 weeks. However, not only leukopenia but also marked G-I toxicities being noted in some cases, care should be taken for those side effects.