Sera from 89 patients with cystic fibrosis (CF) and 88 control persons were examined for the occurrence of rheumatoid factors (RF) of the IgG, IgA and IgM classes by an indirect immunofluorescence method and by the latex fixation slide test. The prevalence of RF-IgG was significantly higher (88%) (p less than 0.0005) among the CF patients than among the control persons (7%), while no difference was found between the two groups with regard to RF of the IgA or IgM classes. Fifty-five of the CF patients had chronic Pseudomonas aeruginosa infection in their lungs and two or more precipitins against these bacteria in their sera determined by crossed immunoelectrophoresis. These CF patients did not differ from the 34 CF patients without chronic P. aeruginosa infection, neither with regard to prevalence nor titer of RFs, but there was a positive correlation between the number of P. aeruginosa precipitins in the 55 chronically infected CF patients and their titers of IgG-RF. Nineteen CF patients were examined also for RFs, antinuclear antibodies (ANA) and anti-DNA antibodies in their sputum sol phase and corresponding sera. RFs were demonstrated in the sputum sol phase from 6 of the patients by the latex fixation test, whereas their sera were negative in this test, possibly indicating a local production of RF. Positive reactions for ANA and anti-DNA antibodies were found in 7 and 10 of the sputa respectively, and in higher titers than in the corresponding sera, also suggesting a local production. Titers of autoantibodies in sputum were low and no difference was found between patients with chronic P. aeruginosa infection and patients without P. aeruginosa infection. The possible role of autoantibodies in the patogenesis of pulmonary tissue damage in CF patients is discussed.