Influenza virus infects airway epithelial cells, resulting in virus replication, amplification and spread, ultimately causing severe disease. Influenza virus also infects airway immune cells, including airway macrophages. We have previously shown that infection of macrophages by seasonal influenza virus is blocked at a late stage in the viral replication cycle, and infectious progeny are not released. Therefore, macrophages potently “block” the growth of influenza virus during infection, limiting disease severity. Consistent with this, some highly pathogenic avian influenza H5N1 strains replicate productively in macrophages. Viral infection of different cell types induces a unique spectrum of host defence genes, including interferon-stimulated genes (ISGs) and genes encoding other proteins with antiviral potential. While studies have identified a number of host proteins in epithelial cells that can interfere with the efficiency of influenza virus infection (hereafter referred to as ‘restriction factors’), the restriction factors that potently block influenza virus replication in macrophages remained undefined. The interferon-induced transmembrane (IFITM) family of restriction factors (IFITM3 in particular) are known for their ability to mediate potent antiviral activity against influenza virus in epithelial cells. The role of IFITMs in restricting influenza virus replication in immune cells has not been addressed. In this study, we used macrophages from mice with deficiencies in interferon regulatory factors (IRF3/7 -/-), interferon signaling (IFNAR2 -/-) and IFITM3 (IFITM3 -/-) to show that susceptibility to influenza virus infection was significantly enhanced 8 hours post-infection. Thus, elicitation of ISGs, including IFITM3, confers potent antiviral activity against the early stages of influenza virus replication. Nevertheless, infectious viral progeny were not released from influenza virus - infected IFITM3-/- macrophages, indicating that additional ISGs or other restriction factors control the latter stages viral replication. Investigations are currently underway to understand the mechanisms and host cell factors controlling the late stages of influenza virus replication in macrophages.