Mitoxantrone, a DNA intercalator, is an effective antitumor drug known to interfere with topoisomerase II function through stimulation of enzyme-mediated DNA cleavage. To clarify the drug structural requirements for stimulation of topoisomerase II DNA cleavage, the cytotoxic activity and molecular effects of mitoxantrone, ametantrone, and a new derivative (BBR2577), bearing a modification on one of the side chains, were examined in relation to their DNA binding affinities and modes of drug-DNA interaction. The results showed a good correlation between cytotoxicity and topoisomerase II DNA cleavage. The modification of one side chain did not influence the cytotoxic potency or the ability of the drug to stimulate DNA cleavage. In contrast, removal of the hydroxyl substituents in the planar aromatic moiety (ametantrone) markedly affected the efficacy of the drug. Ametantrone showed a markedly lower capacity, compared with the other two compounds, to induce cleavable complexes both in intact cells and in SV40 DNA, which suggests a critical role of these substituents in the formation of the ternary topoisomerase II-DNA-drug complex. The poor efficacy of ametantrone is likely due to low stability of the ternary complex. This is possibly related to a different orientation of the drug chromophore intercalated into DNA, compared with those of mitoxantrone and BBR2577. The DNA cleavage efficiencies of the tested drugs at low concentrations correlated with the DNA binding affinity. Identical DNA cleavage patterns were observed with the three compounds, which suggests that all tested drugs share a similar specificity for interaction with sites recognized by the enzyme.