DMP 840 ((R,R)-2,2'-[1,2-ethanediylbis[imino(1-methyl-2, 1-ethanediyl)]-bis(5-nitro-1H-benz[de]isoquinoline-1,3(2H)-dione] dimethanesulfonate) is a novel bis(naphthalimide) that has shown promising antitumor activity in a variety of preclinical model systems. The compound binds to DNA with high affinity and intercalates, but the mechanism of cell killing has not been elucidated. We have used yeast strains to test whether DMP-840 is active against either topoisomerase I or II. We found that temperature-sensitive top2 mutants resistant to etoposide or amsacrine also confer resistance to DMP-840. In addition, cells overexpressing yeast topoisomerase II were hypersensitive to the drug. By contrast, top1 deletions rendered cells hypersensitive to the drug. These results strongly suggest that DMP-840 acts against eukaryotic topoisomerase II and kills cells by converting the enzyme into a cellular poison. We verified that DMP-840 is active against eukaryotic topoisomerase II by demonstrating that the drug stimulates formation of a cleavage complex with purified yeast topoisomerase II in vitro. We also demonstrated that the drug is active against human topoisomerase II by showing that expression of human topoisomerase II restored sensitivity of resistant yeast cells to DMP-840. We have also directly demonstrated that DMP-840 acts as a poison against purified human topoisomerase II alpha. Taken together, these results indicate that DMP-840 acts like other intercalating topoisomerase II poisons; it kills eukaryotic cells by stabilizing the cleavage complex of topoisomerase II with DNA.