We specifically investigated the significance of antibodies directed against pure preparations of lipopolysaccharide antigens and also against elastase of Pseudomonas aeruginosa. Antibodies were detected by an indirect enzyme-linked immunosorbent assay technique both in serum (IgG) and in sputum (sIgA). Patients with cystic fibrosis (CF) chronically infected with Ps. aeruginosa had significantly higher antibody titers both in serum and sputum than CF patients or non-CF persons not colonized with Ps. aeruginosa. There were differences in the antibody spectrum between the colonized CF group and the two non-colonized control group. The sensitivity was highest for homologous O-specific IgG in serum (91.9%), followed by homologous O-specific sIgA in sputum (79.4%) and sIgA anti-elastase in sputum (66.2%). O-specific IgG antibodies in serum indicate a previous contact with various O-antigens of Ps. aeruginosa rather than reflect the present condition of the patient. In the evaluation of the current status sIgA O-specific antibodies in sputum are an appropriate indicator, with titers increasing during acute exacerbations and decreasing to normal values subsequently.