Six patients with relapsed or refractory acute leukemia were treated with 9 mg/m2 or 11 mg/m2 of acivicin daily for seven days in a phase I-II trial. No responses were attained and further dose escalation was prohibited by neurotoxicity in 2 of 3 patients who received 11 mg/m2/day. Although acivicin appears to have limited potential as a single agent, laboratory evaluation of leukemic blasts in one patient revealed cell cycle (S-phase accumulation) and metabolic effects which suggest that acivicin may be effective as a modulator of other antileukemia agents such as cytosine arabinoside.