Guinea pig airways isolated 4, 8 and 16 days (but not 2 days) after intra-tracheal inoculation with parainfluenza-3 virus In Vivo respond hyperreactive to contractile stimuli. In the present study it is shown that the virus-induced airway hyperresponsiveness is associated with a decreased Superoxide production by broncho-alveolar cells after additional stimulation with parainfluenza-3 virus.