The aim of this research is to develop a human/murine chimeric Fab antibody which neutralizes the anthrax toxin, protective antigen (PA). The chimeric Fab was constructed using variable regions of murine anti-PA monoclonal antibody in combination with constant regions of human IgG. The chimeric PA6-Fab was expressed in E. coli. BL21 and evaluated by ELISA and co-immunoprecipitation-mass spectra. The potency of PA6-Fab to neutralize LeTx was examined in J774A. 1 cell viability in vitro and in Fisher 344 rats in vivo. The PA6-Fab did not have domain similarity corresponding to the current anti PA mAbs, but specifically bound to anthrax PA at an affinity of 1.76 nM, and was able to neutralize LeTx in vitro and protected 56.9% cells at 20 aeg/mL against anthrax LeTx. One hundred mu g PA6-Fab could neutralize 300 mu g LeTx in vivo. The PA6-Fab has potential as a therapeutic mAb for treatment of anthrax.