Dexrazoxane (ICRF-187), which is clinically used to reduce doxorubicin-induced cardiotoxicity, has cell growth Inhibitory properties through its ability to inhibit the catalytic activity of DNA topoisomerase II. A study was undertaken to Investigate whether preincubating Chinese hamster ovary cells (CHO) with dexrazoxane prior to camptothecin treatment resulted in potentiation. Camptothecin is a DNA topoisomerase I poison. It was found that pretreating CHO cells with concentrations of dexrazoxane sufficient to strongly inhibit topoisomerase II for periods from 18 to 96 h resulted in significant antagonism of camptothecin-mediated growth inhibition. Lower concentrations that were sufficient to cause partial Inhibition of topoisomerase II and partial dexrazoxane-mediated cell growth inhibition had little effect on camptothecin-mediated growth inhibition. Neither topoisomerase I protein levels nor camptothecin-induced topolsomerase I-DNA covatent complexes were affected by dexrazoxane concentrations that were sufficient to cause antagonism of camptothecin-induced growth Inhibition. However, under these experimental conditions, dexrazoxane caused a decrease in DNA synthesis. Therefore, results presented here confirm the importance of the DNA synthesis-dependent replication fork Interaction with topoisomerase I-DNA covalent complexes for the expression of camptothecin activity. It is concluded that dexrazoxane and camptothecin analogs should be used with caution in combination chemotherapy.