Neutralizing antibodies can protect against challenge with HIV-1 in vivo if present at appropriate concentrations at the time of viral challenge, but any role in the control of established infection is unclear. Here, we show that high serum concentrations of neutralizing monoclonal antibodies, either singly or as a cocktail, have little sustained effect on viral load in established HIV-1 infection in hu-PBL-SCID mice. In some instances, virus replication of neutralization-sensitive virus continues even in the presence of high levels of neutralizing antibody. In most instances, neutralization escape occurs in a few days, even from a cocktail of three antibodies that recognize distinct epitopes. The results imply that humoral immunity is unlikely to play a significant role in the control of established HIV-1 infection in humans.