ICRF-187, the enantiomer of ICRF-159 formulated for iv use, was administered to 23 patients in a phase I clinical trial. The dose ranged from 500 to 1500 mg/m2 every day for 3 days and was repeated every 28 days. The toxic effects included moderate to severe leukopenia and thrombocytopenia, which recovered by the 21st day of a treatment cycle. Myelosuppression was more severe in patients with prior nitrosourea treatment. Nonmyelosuppressive toxic effects included nausea and vomiting, transient elevations in liver function tests, alopecia, and increased urinary clearances of iron and zinc. The starting dose for phase II trials should be 1250 mg/m2 daily for 3 days repeated at 21-day intervals.