Live attenuated influenza A virus (R.I.T. 4050) was administered to 12 healthy subjects and to 13 asthmatics by the use of nasal drops alone. Serum antibody level to influenza A virus (Victoria 3/75) was measured before, 14 and 28 days after the virus inoculation by a single radial haemolysis in gel test (HIG-test). In order to study the effect of influenza infection upon bronchial reactivity, 1.6 per cent solution of histamine diphosphate was administered by the use of a DeVilbiss nebulizer No 40 for ten tidal breaths before, and 2, 3 and 14 days after the virus inoculation. Ten of the 12 healthy subjects displayed a significant rise in the HIG-test. No change was apparent in the bronchial reactivity to 10 breaths of 1.6% histamine in these healthy subjects. Out of seven of the asthmatics with low antibody levels, six displayed a significant increase between 14 and 28 days subsequent to inoculation. Six of the asthmatics had the initial HIG-levels so high before inoculation that infection could not occur, and they acted as controls. In six asthmatics with serological infection, the histamine-induced change in specific airway conductance increased significantly three days subsequent to the virus inoculation. The increased response to histamine was prevented by the prior administration of both salbutamol (Ventoline, Glaxo), and ipratropium bromide (Atrovent, Boehringer-Ingelheim). In six asthmatics with high HIG-titre before virus inoculation no change was apparent in the bronchial histamine reactivity subsequent to inoculation.