Influenza virus-infected cells undergo apoptosis and become susceptible to phagocytosis by macrophages, and this leads to the inhibition of virus propagation in vitro. To assess if this were also true in vivo, mice infected with influenza A/WSN (H1N1) virus were administered with phagocytosis inhibitors and examined for the progress of influenza. Administration of the inhibitors caused a decrease in the level of phagocytosis observed with bronchoalveolar lavage cells. We found that both the lethality in mice and the extent of inflammation in the lung were augmented in those mice. These results suggest that phagocytosis of virus-infected cells helps suppress the progress of influenza in mice.