Abstract This work shows that viral neutralization appears to be a function not only of antibody concentration but also of affinity, valence, and possibly molecular configuration of antibody. When antibody molecules of similar valence are grouped, neutralization increases with increasing molecular affinity. Conversely, if antibody molecules have comparable affinities, an increase in valence markedly increases neutralization. From these studies it appears that valency is perhaps the greater factor contributing to the neutralizing efficiency of antibody molecules. These findings also seem to indicate that the phage neutralization assay may not be accurate for quantitative antibody comparisons.