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We have examined the activities of two novel aza-anthracene-9,10-diones (aza), 1-aza and 2-aza, in HL-60 human leukemia cell lines containing type II topoisomerases with different sensitivities to inhibition by other intercalating agents. The sensitive line, HL-60, was sensitive to 2-aza but not to 1-aza, whereas the resistant HL-60/AMSA was sensitive to neither agent. Measurements of 1- and 2-aza-induced, topoisomerase II-mediated DNA cross-linking in the cells revealed patterns of resistance and sensitivity that paralleled the results in the cytotoxicity assays. However, measurements of drug-induced topoisomerase II-mediated DNA cross-linking using purified HL-60 and HL-60/AMSA topoisomerase II indicated that both agents could stabilize a covalent complex between DNA and the HL-60 enzyme. HL-60/AMSA topoisomerase II resisted stabilization by either agent. This suggests that the resistance of HL-60 cells to 1-aza is not due to the inability of this drug to inhibit topoisomerase II but rather to another, undefined mechanism.