A multicentre study of the effects of influenza virus RIT 4050 (H3N2) in patients with chronic bronchitis was conducted by members of an MRC Committee. The results showed that RIT 4050 vaccine virus did not cause a deterioration in clinical or physiological status in these patients within the limitation of the relatively reproducible ventilatory tests which were employed. This conclusion applied equally to those who were inoculated and became infected and to those who failed to develop serological evidence of infection. The relatively high proportion of antibody-negative volunteers not infected by the inoculated virus raises doubts concerning the value of this method of immunization for patients at risk during influenza epidemics.