A phase I study of ICRF-187 as a 2-hour iv infusion daily for 3 days was conducted in 46 evaluable pediatric patients. The maximum tolerated dose was 3500 mg/m2/day X 3 based on changes in hepatic function and coagulation abnormalities encountered when larger dosages were administered. One patient with acute lymphocytic leukemia achieved a complete remission and four cleared the blood of lymphoblasts. No other objective responses were seen. Pharmacokinetic analysis showed that the children had a larger volume of distribution per kilogram of body weight in the central compartment and total body and a more rapid total-body clearance than adults. These parameters can explain only part of the increased tolerance of children to ICRF-187.