Mice (n = 3) were intranasally infected with a lethal dose of influenza virus mixed with 2-fold diluted immune sera or naive sera. Immune sera collected from vaccinated mice after 3rd immunization were incubated with influenza viruses, A/PR/8/34 (A/PR8, H1N1) (A,B), reassortants A/Vietnam/1203/2004 (A/VN1203, rgH5N1) (C, D), and A/California/04/2009 (A/CA04, H1N1) (E, F). Body weight (A, C, E) and survival rates (B, D, F) were monitored for 14 days. The results were reproducible from two independent experiments.