During DNA replication, the presence of oligonucleotides with partial homology to the template strand was shown to induce a switch of the polymerase from the normal template to the oligonucleotide. The latter acted as a dead-end template and led to abortive replication. The only prerequisite was that the oligonucleotide could form 7-9 base pairs with the newly synthesized DNA strand in order to switch templates. The switch occurred when base pairing of the oligonucleotide could take place with the 3'-end of the newly synthesized strand. These results show that oligonucleotides used in antisense or antigene strategies could have unexpected effects on replication. In addition, oligonucleotide-directed abortive replication might play an inhibitory role during PCR experiments on long DNA templates and lead to the amplification of truncated fragments.