The bovine antibody BLV1H12, which has an ultralong CDR3H, provides a novel scaffold for engineering new functions into the antibody's variable region. By modifying the β-strand "stalk" of BLV1H12 with sequences derived from natural or synthetic protease inhibitors, we have generated antibodies that inhibit bovine trypsin and human neutrophil elastase (HNE) with low nanomolar affinities. We were also able to generate a humanized variant using a human immunoglobulin scaffold that shares a high degree of homology with BLV1H12. Further optimization yielded a highly selective humanized anti-HNE antibody with sub-nanomolar affinity. This work demonstrates a novel strategy for generating antibodies with potent and selective inhibitory activities against extracellular proteases involved in human disease.