The principal investigator proposes to employ a pathogenic chimeric simian-human immunodeficiency virus (SHIV) to determine whether passive immunization can protect against mucosa (vaginal) or needle stick exposure to HIV. Two sources of SHIV/HIV-neutralizing antibody will be evaluated: a monoclonal anti-HIV gp120 (mAb b12) or polyconal anti-SHIV pooled sera from infected rhesus macaques with low virus burdens. Passive immunotherapy will be initiated either 12 hours pre- or 12 hours post intravaginal SHIV exposure and 10 pre- or 1 or 12 hours post subcutaneous injection. The establishment of infection and disease progression will then be monitored over a 12 month period and compared with animals not receiving passive immunization. The results of these studies are designed to establish whether passive immunization with virus neutralizing antibody can prevent or abort SHIV infection or influence the progression of SHIV disease.