For each virus, four groups of 10 mice were inoculated intranasally with 3 LD50 of A/Wilson Smith/33 or A/ Vietnam/1203/2004 virus in a 50 ml volume. At 24 h after inoculation, graded doses (10, 1, 0.1 mg/Kg) of PN-SIA49 or the control antibody (e137, 10 mg/Kg) were administrated to mice by intraperitoneal injection in a final volume of 0.2 ml. A subset of six mice in each group were weighed on the day of virus challenge and then observed and weighed every 2 days for 2 weeks after inoculation. Mice that lost more than 25% of their initial body weight were euthanized.