It was found that 1) morpholinyldoxorubicin, cyanomorpholinyldoxorubicin, and Actinomycin D (but not doxorubicin) stimulated DNA topoisomerase I-induced cleavage at specific DNA sites; 2) only doxorubicin and Actinomycin D stimulated DNA cleavage by DNA topoisomerase II; 3) at higher drug concentrations, DNA intercalators suppressed enzyme-mediated DNA cleavage induced by DNA topoisomerase I, as well as topoisomerase II; 4) only cyanomorpholinyldoxorubicin produced DNA-DNA cross-links; no DNA unwinding could be observed; and 5) DNA intercalation (unwinding) potency of morpholinyldoxorubicin was about 2-fold less than that of doxorubicin.