Yeast Saccharomyces cerevisiae strains that are permeable to the antitumor alkaloid camptothecin are killed by the drug if they express DNA topoisomerase I, the cellular target of the drug (J. Nitiss and J.C. Wang, Proc. Natl. Acad. Sci. USA, 85: 7501-7505, 1988). We show that in a yeast strain permeable to camptothecin but lacking DNA topoisomerase I, sensitivity to the drug was restored upon expression of human DNA topoisomerase I from a plasmid-borne human complementary DNA clone. When the human enzyme was expressed from a galactose-inducible, glucose-repressible yeast promoter, PGAL1, sensitivity to camptothecin was observed in the presence of galactose but not in the presence of glucose. Expression of human DNA topoisomerase I in Escherichia coli was also demonstrated by the complementation of a conditional lethal E. coli DNA topoisomerase I mutant. These systems can be used in the study of human DNA topoisomerase I-camptothecin interactions and in the screening of additional therapeutics targeting the enzyme.