SummaryAntibodies in cystic fibrosis (CF) sera to Pseudomonas aeruginosa mucoid exopolysaccharide and to sodium alginate (a polysaccharide from seaweed chemically similar to mucoid exopolysaccharide) were measured in sera of CF patients to determine if the exopolysaccharide is immunogenic. An enzyme-linked immunosorbent assay was used to test sera from 26 CF patients (18 colonized with pseudomonas and eight non-colonized) and 26 healthy controls. CF patients colonized with pseudomonas had more antibody to mucoid exopolysaccharide (P=0.0008) and to sodium alginate (P=0.0008) than did non-colonized CF patients. Virtually none was found in healthy controls. Duration of colonization was correlated with the level of antibody to sodium alginate (P=0.003) but not with antibody to mucoid exopolysaccharide. Mucoid exopolysaccharide is immunogenic in patients with CF.