H5 HA-specific monoclonal antibodies block hemagglutination by phage particles decorated with H5 HA. Phage particles (H5 or WT ), or influenza virus virions were incubated with 100, 40, or 1 ng of Mab 9E8 or 9B11 for 1 h at room temperature. Hemagglutination was then tested using chicken RBC.