A majority of the neutralizing antibodies bound to highly conserved epitopes in both the HA stalk and head regions A high frequency of the HA-specific antibodies was able to neutralize the virus in vitro (totaling 73% or 11/15; Fig. 3 A) . These neutralizing antibodies could be further categorized into two distinct groups: (a) neutralizing antibodies that displayed hemagglutination inhibition (HAI) activity (HAI + ) and (b) neutralizing antibodies that had no HAI activity, Fig. S2 A) . The antibodies are grouped based on whether they show HAI and/or neutralizing (neut) function. Antibody 1009-3B06 was only tested for binding to whole virus, as this antibody did not bind to rHA due to binding of a quaternary or conformationally sensitive epitope that is not present in the recombinant protein. HAI and neutralization assays were performed in duplicate and repeated at least three times. ELISA curves are provided in Fig. S2 A. (B) ELISA binding as shown by minimum positive concentration (defined for Fig. 2 ) of neutralizing mAbs to rHA or whole virions from pandemic H1N1 or other influenza strains (ELISA binding curves are provided in Fig. S2 A) . Three binding patterns (epitopes 1 and 2, and 3) were observed that coincided with specificity comparisons by competitive ELISA, as illustrated in Fig. 4 A. (C) Three representative neutralizing antibodies (EM-4C04, 70-1F02, and 1009-3B06) were used for HAI and microneutralization (MN) activity against pandemic H1N1 and several other annual or laboratory H1N1 influenza strains. Experiments were performed in duplicates and repeated at least three times. Minimum effective concentration is shown for both assays.