Complementary DNA of the anti-bovine ribonuclease A monoclonal antibody 3A21 variable regions was cloned, sequenced and expressed as a single chain Fv phage antibody to investigate antibody-antigen interaction. The results of amino acid replacement experiments and changes in the affinity constant of the antibody 3A21 with change in pH, indicated that at least one electrostatic interaction participates in this antigen-antibody interaction.