FIG 4.

Infection of BALB/c mice with PB2 mutant viruses. Thirteen mice were infected with 10¹ PFU i.n. (or treated with PBS as a control) and monitored for weight loss (A) and survival (B) for 14 days. At day 3 p.i., three mice in each group were sacrificed for determination of virus titers in tissue samples (C). Viral titers in organ homogenates were determined by plaque assay. Standard deviations represent the titers of three mouse organs. An asterisks indicates a P value of <0.05 (Student's t test).