DNA topoisomerase I releases torsional stress generated in chromatin during transcription and replication. Usually topoisomerase I is recognized to work as a monomer, but previously we have shown that two molecules can form a dimer-like protein-protein complex on a 'suicide' DNA substrate resulting in a topoisomerase I double cleavage complex. Here we show that during the normal relaxation reaction a considerable fraction of human topoisomerase I formed transient dimers on plasmid DNA too. Recombinant as well as topoisomerase I purified from human cells formed double cleavage complexes within a distance of 12 or 14 nucleotides. When topoisomerase I was isolated from camptothecin-treated HeLa cells, a considerable fraction migrated to the same position as topoisomerase I bearing a covalently bound 12-to-14-mer oligonucleotide. Taken together our data suggest that human topoisomerase I double cleavage complexes are part of the normal catalytic cycle of this enzyme that occur in vitro and possibly also in vivo. (c) 2006 Elsevier Inc. All rights reserved.