Aged C57BL/6 mice show reduced NP responses when infected with an influenza virus lacking the PA epitope. Young and aged C57BL/6 mice were intranasally infected with PA-deficient influenza virus (600 EID₅₀) and lung tissue was harvested 10 d later. The frequencies (A) and total numbers (B) of CD8 T cells positive for NP (NP_366-374/D^b) in the lungs of individual infected mice are shown. Bars indicate the medians calculated from data compiled from two independent experiments (total of 8 young and 15 aged mice). Significance was assessed using the Mann-Whitney rank test (two-tailed, 95% confidence).