Mitoxantrone, 1,4-dihydroxy-5,8-bis[[2-[(2-hydroxyethyl)amino]ethylamino]]9,10 anthracenedione dihydrochloride, CL 232,215 (NSC 301739) is a synthetic aminoanthraquinone which binds to DNA (2, 17), and was designed to reduce or eliminate the cardiotoxicity seen with the structurally related anthracycline antibiotics (1). Preclinical studies have shown that mitoxantrone is active both in vitro and in vivo, against a variety of experimental murine leukemias, including P388 and L 12 10. P388/ADR, a subline resistant to doxorubicin, was found to be partially resistant to mitoxantrone (1, 9, 10, 16). The Mount Sinai phase I study of this novel compound investigated a daily x 5 schedule of administration in patients with carcinomas and sarcomas (6), and showed that mitoxantrone was well tolerated without toxic side effects, except bone marrow suppression, at the recommended dose of 6.8 mg/m2 daily x 5. In order to evaluate the effects of mitoxantrone on leukemic bone marrow, a phase I-II study of the drug was undertaken in patients with acute leukemia in relapse.