Tests for precipitins against Staph. aureus, H. influenzae, Ps. aeruginosa, Strep. pneumoniae, and Kleb. pneumoniae were carried out on the serum of 195 patients with cystic fibrosis, whose ages ranged from 3 weeks to 31 years. Sputum was obtained for culture from 96 patients over 5 years old. Precipitins against Ps. aeruginosa were more common than those against any other organism, including Staph. aureus, in the 0- to 5- and 6- to 10- year-old groups, while mucoid Ps. aeruginosa was the commonest pathogen isolated from the sputum of the 6- to 10-year-old children. These findings suggest that Ps. aeruginosa is the commonest bronchial pathogen in the younger patients and seem to conflict with the belief that Staph. aureus is always the initial pathogen in cystic fibrosis. Indeed, in 2 patients Ps. aeruginosa was proved to be the initial pathogen, and it is probable that patients with cystic fibrosis are susceptible from birth to bronchial infection by any pathogen with which they come in contact. The prevalence of pseudomonas infection, in contrast to that for staphylococcal or haemophilus infections, fell strikingly in the oldest patients, and this may reflect the failure of many patients with pseudomonas infections to survive into adult life.