Intercalator-induced DNA double-strand breaks (DSB) presumably represent topoisomerase II DNA cleavage sites in mammalian cells. Isolated L1210 cell nuclei were used to determine the saturability of this reaction at high drug concentrations. 4'-(9-Acridinylamino)methanesulfon-m-anisidide (m-AMSA) and 5-iminodaunorubicin (5-ID) both produced DSB in a concentration-dependent manner, and the production of these breaks leveled off above 10 microM. Addition of m-AMSA to 5-ID-treated nuclei did not raise the plateau level. Thus, both drugs seemed to interact similarly on identical targets. The ellipticine derivative 2-methyl-9-hydroxyellipticinium (2-Me-9-OH-E+) had two effects on the production of DSB. Below 10 microM, 2-Me-9-OH-E+ produced DSB as did ellipticine, m-AMSA, or 5-ID. Above 10 microM, 2-Me-9-OH-E+ did not induce DSB and inhibited the DSB induced by m-AMSA, 5-ID, or ellipticine. 2-Me-9-OH-E+ and m-AMSA competed with each other to produce either double-strand break formation (m-AMSA-induced reaction) or double-strand break inhibition (2-Me-9-OH-E+-induced reaction at concentrations greater than 10 microM). Because these results were reproduced in experiments using DNA topoisomerase II isolated from L1210 nuclei, it is likely that the intercalator-induced protein-associated DNA breaks detected by alkaline elution in nuclei represent DNA topoisomerase II-DNA complexes.(ABSTRACT TRUNCATED AT 250 WORDS)