The sera from 32 patients with cystic fibrosis who were chronically colonized with mucoid strains ofPseudomonas aeruginosa(MPA) were tested for anti-MPA antibodies. Using an enzyme-linked immunosorbent assay we measured IgA, IgG, and IgM antibody titers to three MPA strains, extracts of those strains, and seaweed-derived sodium alginate, which is similar chemically to the exopolysaccharide of MPA. These titers were compared with identical tests performed on the sera of eight cystic fibrosis patients who never were colonized with MPA and 10 normal adults. The IgG titers were significantly higher in tests of sera from the colonized patients compared with the other two groups but the IgA and IgM titers were not significantly higher. In colonized patients antibody titers to the different antigens correlated with each other suggesting that the major antibody response was to common antigenic determinants. Using these titers as a data base, eight patients whose clinical status was unknown to the testers, had IgG-enzyme-linked immunosorbent assay tests of their sera and the four colonized patients with cystic fibrosis were correctly identified. Three of them had substantial titers of antibody in the bronchoalveolar lavage fluid.