In summary, we successfully established an ARDS mouse model induced by a virulent 2009 H1N1 variant, which demonstrated key human ARDS clinical and pathological features, such as respiratory distress, low Pa O2 , exudative, proliferative and fibrotic lung, and high levels of inflammatory cells and cytokines. The mouse model may contribute to the study of the pathogenesis and therapy of ARDS induced by 2009 H1N1 virus. To determine LD 50 of SD/09 virus, eight 6-week-old female BALB/c mice per group were inoculated i.n. with 10 1 -10 6 pfu (50 ml) viruses and monitored for 14 days. The value of MLD50 was calculated using the Spearman-Karber method and expressed by pfu per MLD 50 [39] .