The mortality rate associated with inhalation anthrax is high, even if antibiotics are administered in a timely manner. Renewed fears that Bacillus anthracis spores may be employed as a biological threat agent have prompted a search for an adjunct therapy that might lower mortality rates in persons known to be exposed. Several lines of evidence suggest that toxin-specific antibodies, acquired either through vaccination or administered passively, work synergistically with antibiotics in post-exposure therapy for inhalation anthrax. We have successfully cloned and expressed the majority of antibodies that comprise the human antibody repertoire specific for the protective antigen (PA) of B. anthracis using B cells from AVA-vaccinated donors. From this collection of fully human, PA-specific monoclonal binding domains, we propose to establish a panel of neutralizing antibodies to be used as an engineered "polyclonal" antibody-based therapeutic to neutralize 8. anthracis toxins. Specifically, we will 1) convert all previously isolated PA-specific antibody clones to a format suitable for expression in eukaryotic systems, 2) identify those paratopes that neutralize toxin in an in vitro cell protection assay, 3) identify a subset of non-competing PA-neutralizing clones, 4) optimize antibody expression both in our bi-cistronic pARC/IRES expression vectors and the established Lonza glutamine synthetase-based antibody expression system. We anticipate a product ready for in vivo testing in an animal model system at the conclusion of the proposed period of support. The PA-specific panel we construct will be molecularly defined and characterized, incorporate the advantages of toxin-specific, polyclonal human sera, and lack the inherent risk associated with blood derived products.