Neutralization titration curve of macaque Fabs against SIVsmH4. The Fab concentration is plotted versus the resulting protection from virus-induced killing in CEM×174 cells. Two Fabs (201 and 202) neutralized SIV in a dose-dependent fashion, whereas all other Fabs, including two irrelevant Fabs, 1LNRh544 and 2LNRh544 (negative [neg] controls); from the unscreened library, failed to neutralize SIV, even at high concentrations.