The concentrations of nine plasma proteins were determined by quantitative immunoelectrophoresis in sputum specimens from 29 patients with cystic fibrosis (CF) and from 24 patients with severe asthma and chronic bronchitis. The results suggested that the population of CF patients could be divided into two groups in spite of an absence of difference in clinical status between the groups. Average concentrations of seven plasma proteins in sputum of group I CF patients were identical with those in sputum of patients with bronchitis, but the average concentrations of six of these proteins in sputum from group II CF patients were higher than those in specimens from the bronchitic patients and were similar to corresponding concentrations in sputum from patients with asthma, all of whom were examined while in status asthmaticus. The average concentrations of 14 secretory proteins were the same in all sputum specimens whether or not they were produced by patients with cystic fibrosis, asthma or bronchitis. It was concluded that the concentrations in the bronchopulmonary secretions of proteins associated with host defence were not diminished in patients with cystic fibrosis, and failure to produce adequate concentrations of proteins with antimicrobial activity was unlikely to be responsible for the above average susceptibility to chest infection in cystic fibrosis. It is suggested that there exists a group of CF patients in whom a pulmonary allergic reaction generates an inflammatory response as severe as that characterizing status asthmaticus and that this response could be detrimental.