Cystic fibrosis (CF) patients who are chronically infected with Pseudomonas aeruginosa make serum antibodies to bacterial surface LPS as well as other Pseudomonas antigens. This study investigated the feasibility of using oral fluid samples for the detection of Pseudomonas antibodies in CF patients and compared these results with corresponding serum antibodies. Most strains of P. aeruginosa produce two forms of LPS molecule, termed A-band (described as a common antigen) and B-band (O-serotype-specific antigen), apparently bound to a common core oligosaccharide moiety. A-band LPS was demonstrated in 45 out of 49 clinical isolates of P. aeruginosa by SDS-PAGE and immunoblotting with a specific antibody. Oral fluids were collected from 17 adult CF patients, all of whom were sputum culture positive for P. aeruginosa (13 also provided serum samples), 11 primary ciliary dyskinesia (PCD) patients and 37 healthy volunteers. Antibodies to A-band LPS were detected by immunoblotting in all of the CF patients' oral fluids but 10 of the volunteer samples gave weak reactions with immunoblotting. Six of the PCD patients gave a weak reaction with A-band antibodies and only one demonstrated antibodies to core LPS. In a quantitative ELISA, 15 of the 17 CF patients' oral fluids were shown to contain antibodies to A-band LPS, whilst none of the volunteer samples contained antibodies to A-band LPS. All serum samples from the CF patients were positive by both methods. Thus this is a sensitive procedure for the detection of antibodies to A-band LPS of P. aeruginosa in oral fluid and serum from patients with CF.