We describe here the selection of ultra-potent anti-respiratory syncytial virus (RSV) antibodies for preventing RSV infection. A large number of antibody variants derived from Synagis^® (palivizumab), an anti-RSV monoclonal antibody that targets RSV F protein, were generated by a directed evolution approach that allowed convenient manipulation of the binding kinetics. Palivizumab variants with about 100-fold slower dissociation rates or with fivefold faster association rates were identified and tested for their ability to neutralize virus in a microneutralization assay. Our data reveal a major differential effect of the association and dissociation rates on the RSV neutralization, particularly for intact antibodies wherein the association rate plays the predominant role. Furthermore, we found that antibody binding valence also plays a critical role in mediating the viral neutralization through a mechanism that is likely unrelated to antibody size or binding avidity. We applied an iterative mutagenesis approach, and thereafter were able to identify palivizumab Fab variants with up to 1500-fold improvement and palivizumab IgG variants with up to 44-fold improvement in the ability to neutralize RSV. These anti-RSV antibodies likely will offer great clinical potential for RSV immunoprophylaxis. In addition, our findings provide insights into engineering potent antibody therapeutics for other disease targets.