After confirming the expression of sialic acid receptors on progenitor epithelial cells, we next examined the susceptibility of progenitor epithelial cells to mammalian and avian influenza viruses. Cells were inoculated with SwIV, AvIV or HuIV at a MOI of 1 for 1 hour (h). As shown in Fig. 3 , infectious viruses were observed in culture supernatants after 24 h after infection and viral titers slightly increased at 36 h. Among the viruses tested, SwIV produced highest cellular lysis (Fig. 3B ) and replicated to highest titers followed by AvIV and HuIV (Fig. 3C) . The production of cytopathic effects required live, infectious virus because cell cytotoxicity was not observed in heat-inactivated virus-infected cultures (data not shown).