The roles of four VH surface lysine side-chains in antibody recognition of DNA containing a central pyrimidine (6-4) pyrimidone photoproduct was investigated. Three of the four lysines (at positions H62, H64 and H66) are strongly conserved; however, the presence of lysine at position H68 was highly unusual. Each lysine was replaced individually by alanine; in addition, a quadruple mutant was also created. Studies of the binding kinetics revealed that these lysine residues mainly influenced the association phase. Our results suggest that these side chains help to guide the DNA polyanion to the antigen binding pocket by electrostatic effects.