Immunofluorescence staining of viral nucleoprotein (NP) in influenza virus–infected adult and neonatal monocyte-derived macrophages (MDMs). Day-14 differentiated MDMs were infected by influenza viruses H1N1/98, H9N2/G1, and H5N1/97 at an MOI of 2. At the 12-h point of infection, cells were fixed and analyzed by immunofluorescent staining specific for influenza A virus matrix protein and NP (green) with Evan’s blue (red) as a counterstaining. More than 90% of cells were infected