influenza A virus infection and/or vaccinations (H1N1 and H3N2). Other investigators have shown that a low level of heterotypic anti-HA Abs that bind and neutralize H5N1 viruses is present in IVIG from diverse geographic locations [9, 10] . These investigations further showed that these heterosubtypic anti-H5N1 Abs cross-react with H3N2 and H1N1; however, efforts to purify and characterize these Abs (beyond neutralization titers) have not been reported. Our data show that there are 2 populations of heterosubtypic Abs with different HA binding ability in IVIG: one can bind to HAs from both Group 1 and Group 2 viruses; the other is specifically directed against Group 1 stem pocket. Approximately 0.01% of IVIG (most certainly derived from H5-and H7-naive donors), which is purified by acid elution from H5-beads, has heterosubtypic binding activity to both Group 1 and Group 2 HAs. This fraction of IVIG Abs also demonstrated neutralizing activity against the H1N1 and H5N1 pseudotyped viruses. The F10-like stem pocket-directed Abs were also detected in IVIG and were recovered at 10% of the levels of H5 binding Abs. As expected, these F10-like Abs displayed similar binding and neutralization profiles among the tested HAs and viruses as the Group 1 specific mAbs that are directed to the stem pocket of HA [1, [3] [4] [5] . For both Ab fractions, a broader range of other subtypes were not tested due to limited amount of the Abs that we could purify from IVIG (1 lg and 0.1 lg/100 mg IVIG, respectively), larger-scale purification of H5 protein and other materials will be required to characterize these heterosubtypic BnAbs in more detail.