There is concern that variola virus, the causative agent of smallpox which was eradicated as a human pathogen more than two decades ago, forms a threat to humans once again, this time as an agent of bioterrorism. The use of variola virus in a bioterrorist attack would be met by the use of the licensed live vaccinia virus vaccine. This vaccine may cause serious side effects which can be successfully treated with vaccinia immune globulin (VIG) derived from hyperimmune individuals. VIG however is in short supply and future availability is uncertain, and in addition suffers from the general concerns of using human blood products for therapeutic applications. This proposal aims to prepare and characterize human monoclonal antibodies against vaccinia virus which, likely formulated as an antibody cocktail, will constitute a replacement for VIG. We will isolate neutralizing antibodies against both infectious forms of vaccinia virus, i.e. intracellular mature virus (IMV) and extracellular enveloped virus (EEV). We will place particular emphasis on isolating antibodies against EEV, as EEV mediates dissemination of infection and is the viral form against which protective immune responses are directed. Inactivation of vaccinia virus will be studied in vitro and in vivo, and will be aimed at designing an antibody cocktail that provides protection against vaccinia virus infection in pre-exposure and post- exposure immunopropylaxis. The antibody cocktail designed may provide a treatment for smallpox itself. To examine the impact of passive immunization in immunoprophylaxis and immunotherapy of a smallpox-like disease in a non-human primate model, we will use an experimental model of monkeypox virus infection