Given the suboptimal outcome of VRC01 antibody-mediated prevention of HIV-1 infection in its first field trial, means to improve diverse antiviral activitiesin vivohave renewed importance. This work revisits a loss-of-function experiment that investigated the mechanism of action of b12, a similar antibody, and finds that the reason why complement-mediated antiviral activities were not observed to contribute to protection may be the inherent lack of activity of wild-type b12, raising the prospect that this mechanism may contribute in the context of other HIV-specific antibodies.