Antibodies specific for M2 but not NP protect against influenza-induced tachypnea. Groups of 6 animals were inoculated i.p. with 100 μg monoclonal antibody (anti-M2 or anti-NP) prepared in saline solution 24 hr before infection with A/Wuhan/95 (H3N2). Control groups of animals underwent passive transfer of 0.5 ml (i.p.) of serum from H1N1-immune animals, or were either infected with the same H3N2 virus or A/PR/8/34 (H1N1) virus 28 days earlier. The percent protection was calculated from RR measured by whole body plethysmography. Groups of animals that had RR statistically different (p < 0.05) from animals undergoing primary influenza infection are designated in the figure with an *.