Subjects prone to recurrent acute bronchitis were admitted to a six-month double-blind clinical study, in which the value of oral immunization with a preparation of killed Haemophilus influenzae was tested. Most subjects had early smoking-related chronic lung disease, unrecognized by either the patient or his/her doctor. Subjects taking the active agent had a 41% reduction in the total number of episodes of acute bronchitis (P = 0.16), a 60% reduction in the number of episodes of acute wheezy bronchitis (P = 0.02) and a 58% reduction in antibiotic use (P = 0.07). The power of analysis was restricted by the small study group. Parameters of episode severity favoured the test group, suggesting that individual episodes of acute bronchitis in subjects taking an oral preparation of killed H. influenzae were less severe than in those taking placebo tablets. Oral immunization with H. influenzae selectively reduced the increase in colonization of the oropharynx with H. influenzae which occurred in the group taking placebo. This is the first demonstration that the common mucosal system can be activated to modify a colonization pattern at a distant site.