The suckling mice were subcutaneously injected in the neck with 50–100 ug of plasmids dissolved in 100 ul of saline. After 6 h, the suckling mice were challenged with 5 LD50 and 20 LD50 FMDV serotypes Asia I per 0.1 milliliter by subcutaneous injection in the neck near the site that received the injected DNA and were then observed for 5–6 days postchallenge. All saline-injected mice (n _ 10 mice per group) died within 69 h, with most mice dying within 48 h, after the viral challenge. Only 3 of 9 mice pretreated with pSi-FMD2 and 4 of 10 mice pretreated with pSi-FMD3, survived a viral challenge of 5 LD50 for 5 days of observation. Further, only 1 of 9 mice pretreated with pSi-FMD2 and 1 of 9 mice pretreated with pSi-FMD3 survived a viral challenge of 20 LD50 for 5 days of observation.