The ultimate goal of an AIDS vaccine is to elicit potent cellular and humoral immune responses that will result in broadly enduring protective immunity. During the past several years, we have focused on characterizing the quantitative and qualitative properties of the antibody response, principally working to define the mechanism(s) of antibody-mediated neutralization in vitro. We have utilized a panel of monoclonal antibodies generated from monkeys infected with attenuated SIV for more than 8 mo to dissect the early events of virus infection involved in antibody-mediated neutralization. Presented herein are highlights from our studies that have identified potential mechanisms by which antibodies neutralize SIV in vitro.