Mitoxantrone (DAD) is a new agent which intercalates into DNA. Preclinical studies have demonstrated activity equal to or greater than that of doxorubicin in all tumor systems tested. In this phase I clinical trial, the schedule of drug administration consisted of a 24-hour continuous iv infusion repeated at 21-day intervals. Twenty-nine patients received a total of 66 courses over a dose range of 4-15 mg/m2. The dose-limiting toxic effect was leukopenia, with a wbc count nadir on Day 12 and resolution prior to Day 21. Other toxic effects were thrombocytopenia, mild phlebitis, and blue discoloration of veins. Objective tumor responses were seen in a patient with adenocarcinoma of the breast and in another patient with clear cell carcinoma of the vagina. An additional six patients with acute leukemia were treated at a dose of 12 mg/m2; a decrease in peripheral blast count was observed in four of these six patients. The toxicity of DAD by 24-hour iv infusion was similar to that previously reported for iv bolus administration. We recommend phase II evaluation of DAD at a dose of 12 mg/m2 by single iv injection at 21-day intervals. Patients with acute leukemia should be evaluated at higher dose levels.