IgG antibody titers in mice immunized once with various dosages of H5N1 virus HA DNA. Twenty BALB/c mice aged 6–8 weeks were divided into 4 (n = 5) groups. One group of non-immunized mice was set up as a control. The mice in the three test groups were immunized once with 10 μg, 50 μg and 100 μg of HA DNA, respectively. Blood samples were collected on day 3, 5, 7, 10, 14, 21, and 28 after immunization and specific IgG antibodies were detected by ELISA. Ab-positive cut-off values were set as mean + 2 SD of non-immunized sera. An ELISA Ab titer was expressed as the highest serum dilution giving a positive reaction.