In this application is described the production of recombinant, chimeric, humanized antibodies specific for both BoNt/A and BoNT/B. The humanized antibodies were converted from a mouse anti-BoNT/A/B Fab fragment into a whole human IgG1 antibody. The antibodies are useful in assays for detecting human exposure to BoNT/A and BoNT/B.