Since 1982 all infants born within the East Anglian Regional Health Authority have been screened for cystic fibrosis. Between April 1985 and April 1992 infants identified in this way have been entered into a randomised prospective controlled trial of antibiotic prophylaxis. Approximately half the infants received continuous oral flucloxacillin and the remainder received antibiotics when clinically indicated. Infants underwent tests of respiratory function at 3-4 months and at 1 year of age. Measurements of thoracic gas volume and airway conductance were made with an infant whole body plethysmograph, and maximum expiratory flow by the 'squeeze' technique. A total of 73 tests was performed of 42 infants. To facilitate comparisons, measurements were expressed as scores. The mean values of the scores for the two groups of infants fell within normal limits. There was no difference between the treatment groups at either age. A reduction in airways conductance was observed between the two tests.