Specificity of NP antibodies. Immunofluorescence microscopy of HeLa cells infected with the influenza A virus and immunolabeled with the monoclonal NP antibody, or the polyclonal anti-peptide antibodies that recognize the NLS1 and the NLS2 epitopes of NP. DAPI, a DNA marker, was used to determine the total number of cells present. As a control, a mock infection without influenza A virus was also performed. Cells were fixed and prepared for immunofluorescence microscopy 17 hours after infection.