We used purified mammalian topoisomerases I (top1) and oligonucleotides to study top1-mediated cleavage and religation in the presence of a potent carcinogenic adduct, 1,N6-ethenoadenosine (εA) incorporated immediately downstream of a unique top1 cleavage site. We found tha εA markedly enhanced top1 cleavage complexes when it was incorporated at the +1 position of the top1 cleavage. This enhancement was due to a reduction of the religation step of the top1 reaction. In addition, εA reduced the top1-mediated cleavage and decreased binding of the enzyme to DNA. We also studied the effects of the εA adduct on top1 trapping by camptothecin (CPT), a well known top1 inhibitor. CPT was inactive when εA was present at the +1 position. Alkylation of the top1 cleavage complex by 7-chloromethyl-10,11- methylenedioxycamptothecin (7-ClMe-MDO-CPT) was also blocked by the εA adduct. Altogether, these results demonstrate that the εA carcinogenic adduct can efficiently trap human top1 and mimic CPT effects. Normal hydrogen bonding of the base pairs immediately downstream from the top1 cleavage site is probably essential for efficient DNA religation and binding of camptothecins in the top1 cleavage complex.