Aziridinylbenzoquinone is a quinone compound capable of penetrating the central nervous system. It has demonstrated activity against both intracranial and i.p. murine tumors and human tumor xenographs. We have conducted a Phase I trial of aziridinylbenzoquinone in 60 children with advanced cancer who were refractory to conventional therapy. The drug was given by slow i.v. push on a daily schedule for 5 days every 3 to 4 weeks. The dose range explored included 6 dose levels, ranging from 6 to 12 mg/sq m daily for 5 days in patients with solid tumors and leukemia, and in patients with leukemia, 20, 25, and 30 mg/sq m daily for 5 days. Myelosuppression was the dose-limiting side effect. In patients with solid tumor the highest dose studied was 12 mg/sq m, and the median nadir white blood cell and platelet counts were 0.7 X 10(3) and 6.0 X 10(3)/microliter on Days 17 and 22, respectively. The median recovery day for white blood cells was 39. There may be some evidence of cumulative toxicity with prolonged thrombocytopenia. Other side effects were mild nausea, vomiting, and mucositis. Elevations in liver enzymes and bilirubin were transient and dose dependent, occurring 3 to 4 weeks after drug administration. Of the 34 children with solid tumors, 33 were evaluable for hematopoietic toxicity, 3 were early deaths, and 31 receiving a total of 55 courses were evaluable for therapeutic response. Partial responses lasting 3 weeks to 6 months were seen in the 4 patients with Hodgkin's disease, and in a child with a metastatic spinal cord ependymoma. Fifty-two courses were given to 9 patients with acute lymphocytic leukemia and 17 with acute nonlymphoblastic leukemia. Of the 15 patients with acute nonlymphoblastic leukemia treated at doses greater than or equal to 25 mg/sq m/day for 5 days there was one early death and there were 2 M1 (less than or equal to 5% blasts with normal cellularity), 3 M2A (6 to 15% blasts), and 2 M2B (16 to 39% blasts) bone marrow responses lasting 1 to 3.5 months. Aziridinylbenzoquinone demonstrated activity against acute nonlymphocytic leukemia with maximal tolerated doses of 30 mg/sq m daily for 5 days. Its effect in Hodgkin's disease is encouraging; however, further study will be required to determine its efficacy in central nervous system cancers. Recommended doses for Phase II studies, using daily schedule for 5 days in children with solid tumors, is 9 mg/sq m, and in children with leukemia, it is 25 mg/sq m.