Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.