Tyrosine phosphorylation plays a crucial role in cell proliferation and cell transformation which suggests that tyrosine kinase-specific inhibitors might be used as anticancer agents. When the cytotoxic effect of the potent tyrosine kinase inhibitor genistein on various cell lines was studied, we observed that 9-hydroxyellipticine-resistant Chinese hamster lung cells (DC-3F/9-OH-E) were markedly more resistant to genistein than the parental cell line (DC-3F). The DC-3F/9-OH-E cells have been shown to have an altered DNA topoisomerase II activity. We therefore examined the effects of genistein on DNA topoisomerase II-related activities of nuclear extracts from DC-3F cells as well as on purified DNA topoisomerase II from calf thymus. Our results show that genistein (a) inhibits the decatenation activity of DNA topoisomerase II and (b) stimulates DNA topoisomerase II-mediated double strand breaks in pBR322 DNA on sites different from those of 4'-(9-acridinylamino)methanesulfon-m-anisidide, etoposide, and 2-methyl-9-hydroxyellipticinium. Structure-activity studies with six chemically related compounds show that only genistein has an effect on the cleavage activity of DNA topoisomerase II in the concentration range studied. Finally, genistein treatment of DC-3F cells results in the occurrence of protein-linked DNA strand breaks as shown by DNA filter elution. Viscometric (lengthening) studies demonstrate that genistein is not a DNA intercalator. Genistein is therefore an interesting compound because it induces cleavable complexes without intercalation. Taken together, our results show that genistein is an inhibitor of both protein tyrosine kinases and mammalian DNA topoisomerase II. This could be accounted for by the sharing of a common structure sequence between the two proteins at the ATP binding site.