Abstract. Serum from normal persons and from 133 cystic fibrosis patients was examined for precipitins against faecal bacteria (E. coli and B. fragilis) and against P. aeruginosa, by means of crossed immunoelectrophoresis. 27% of the normal sera contained 1–2 precipitins against E. coli, 16% contained one precipitin against B. fragilis and 6% contained one precipitin against P. aeruginosa. In sera from cystic fibrosis patients, there was a significantly increased prevalence of precipitins against E. coli (76%), B. fragilis (38%) and P. aeruginosa (63%), and the mean number of precipitins against each of these bacteria was 3 (range: 1–12), 1.4 (range: 1–4), and 16 (range: 1–60), respectively. Increased numbers of precipitins against P. aeruginosa were correlated with chronic lung infection caused by this species and with poor prognosis, whereas such associations were not revealed as regards precipitins against the two other bacterial species. Increased numbers of precipitins against E. coli and B. fragilis were significantly associated with increased numbers of precipitins against P. aeruginosa. It is suggested that this association is due to immune reactions in the intestinal mucosa, resulting in increased absorption of antigens from the gut. Copyright © 1979, Wiley Blackwell. All rights reserved
