Binding of simian virus 40 (SV40) large T antigen to human and calf thymus topoisomerase I (topo I) was readily detected by using modified enzyme-linked immunosorbent assays and immunoblots. In addition to WT T antigen, binding could also be readily demonstrated with T antigen fragments from the amino-terminal region as well as with fragments missing this region, but much less so with small t antigen or with human p53. Antibody-blocking experiments showed that a monoclonal antibody that binds to the N-terminal region and several antibodies that recognize the central region of T antigen interfere with the binding to topo I. Our data are consistent with the existence of two separate topo I-binding regions in T antigen, one mapping within residues 82 to 246 and an apparently weaker one present after residue 246. By comparing the binding of T antigen to topo I with that of T antigen to DNA polymerase alpha or RPA, a single-stranded DNA-binding protein, it was determined that the T antigen-topo I interaction is much stronger and that the binding sites for topo I and DNA polymerase overlap, whereas the one for RPA differs. Several unwinding-defective mutants of T antigen were partially defective in their binding to topo I, suggesting that the binding to topo I is required for unwinding circular DNA. Finally, immunoprecipitation experiments demonstrated that T antigen can interact with DNA-bound topo I, indicating that such an interaction may take place during SV40 DNA replication.