DNA topoisomerase I (topo I) is a member of a group of essential nuclear enzymes which control and modify the topological state of DNA and is recognized as the target for anticancer drugs. During the course of the catalytic activity of topo I, a covalent bond is formed between a tyrosine group at the active site of the enzyme and a 3' phosphate group along the DNA backbone. This chemical reaction resembles the protein kinase-mediated tyrosine phosphorylation process. We assumed, therefore, that tyrphostins, potent and selective blockers of protein tyrosine kinases, might affect topo I activity. We found that of three derivatives of tyrphostins (AG-555, AG-18, and AG-213) that inhibited topo I activity in an in vitro assay, AG-555 was the most active. Examination of the mechanism by which these compounds act as topo I inhibitors revealed that AG-555 blocked the binding of this enzyme to the DNA due to its interaction with the topo I enzyme. We showed that its mode of action differed from that observed for camptothecin, a known topo I inhibitor. However, AG-555 did not affect the activity of other major DNA binding enzymes (i.e., DNA ligase, DNA polymerase I, and reverse transcriptase). This study suggests that tyrphostins may serve as a new class of topo I inhibitors, and these results also present additional explanations for their antiproliferative effect.