The development of the human immunodeficiency virus-1 (HIV-1)/simian immunodeficiency virus (SIV) chimeric virus macaque model (SHIV) permits the in vivo evaluation of anti-HIV-1 envelope glycoprotein immune responses 1, 2, 3. Using this model, others, and we have shown that passively infused antibody can protect against an intravenous challenge 4, 5. However, HIV-1 is most often transmitted across mucosal surfaces 6, 7, 8, 9 and the intravenous challenge model may not accurately predict the role of antibody in protection against mucosal exposure. After controlling the macaque estrous cycle with progesterone 10, anti-HIV-1 neutralizing monoclonal antibodies 2F5 and 2G12, and HIV immune globulin were tested 11, 12, 13. Whereas all five control monkeys displayed high plasma viremia and rapid CD4 cell decline, 14 antibody-treated macaques were either completely protected against infection or against pathogenic manifestations of SHIV-infection. Infusion of all three antibodies together provided the greatest amount of protection, but a single monoclonal antibody, with modest virus neutralizing activity, was also protective. Compared with our previous intravenous challenge study with the same virus and antibodies 5, the data indicated that greater protection was achieved after vaginal challenge. This study demonstrates that antibodies can affect transmission and subsequent disease course after vaginal SHIV-challenge; the data begin to define the type of antibody response that could play a role in protection against mucosal transmission of HIV-1.