Chronic respiratory infection due to Pseudomonas aeruginosa remains the most important prognostic factor in cystic fibrosis patients. One method to lengthen the patient's life is to extend the initial state of the illness with an early diagnosis, before Ps. aeruginosa infection becomes chronic. Often this is difficult because of the young age of the patients. This study tested an immunoenzymatic system to evaluate antibody response against three Ps. aeruginosa purified antigens, alkaline protease, elastase and exotoxin A. We studied 40 patients with cystic fibrosis, 20 affected and 20 unaffected by apparent Ps. aeruginosa infection, also from the bacteriological point of view. Serological and bacteriological results were compared for each patient and showed that serological screening can be useful in young subjects, who often have no bacteriological evidence of Ps. aeruginosa colonization.