The highest percent of neutralization was seen at a plasma dilution of 1:6. Values in parentheses represent the highest % of neutralization reached for RZu-11 and RRs-11 at plasma dilutions 1:54 and 1:18, respectively. SHIVIG plasma IC50 concentrations were determined using the concentration of SHIVIG in RM plasma on the day of challenge and the dilution of the same plasma sample showing 50% of neutralization in PBMC assay. Neutralization assays were performed at least in triplicate. Measurements of viral loads were performed in duplicate.