Triple helix forming oligonucleotide directed psoralen adducts in a mammalian shuttle vector have been reported to be repaired efficiently in human cells. In this study we examined the role of intermolecular homologous recombination in triple helix targeted psoralen adduct repair. A simian virus 40 (SV40) shuttle vector carrying a mutated supF gene was treated with a triplex forming oligonucleotide psoralen conjugate and cotransfected into human cells with a second plasmid bearing the wild type supF gene. Recombinants with a reactivated marker gene were detected by an X-gal assay in indicator bacteria. We could observe a low frequency of psoralen adduct induced recombination indicating that recombination does not play a major role in triplex directed psoralen adduct repair. The implications for targeted mutagenesis by triple helix forming oligonucleotides are discussed.