A 31P nuclear magnetic resonance (NMR) study of the interactions between oligonucleotides containing the (6-4) photoproduct and the Fab fragments of monoclonal antibodies (64M3 and 64M5) recognizing the (6-4) photoproduct is reported. The 31P chemical shift data indicate that backbone conformation of (64) adduct is affected by the presence of flanking oligodeoxynucleotides, and (6-4) adducts with different backbone conformations are accommodated in the antigen binding sites of these antibodies. It was also revealed that epitopes for these antibodies consist of not only the (6-4) adduct but the flanking di- or tri-deoxynucleotides on both the 5' and 3' sides as well.