Sera from 84 patients with cystic fibrosis, 31 patients with other respiratory diseases and 21 control persons have been investigated for the occurrence of antibacterial precipitins and autoantibodies. Preciptins were studied by means of crossed immunoelectrophoresis, antinuclear factors by an indirect immunofluorescence technique and rheumatoid factors by the latex fixation slide test. A very heterogeneous antibacterial immune response was found in patients with cystic fibrosis, notably as regards Ps. aeruginosa. None of the other patients or controls had precipitins against this bacterium. The occurrence of precipitins against St. aureus and D. pneumoniae were more frequent in cystic fibrosis patients as compared with controls, but not as compared with other respiratory disease patients. No significant differences were found as regards precipitins against H. influenzae or the occurrence of rheumatoid factors. Antinuclear factors were more frequent in cystic fibrosis patients than in the other two groups investigated. A pronounced and heterogeneous humoral immune response against Ps. aeruginosa in cystic fibrosis patients chronically infected with mucoid strains of this bacterium was found to be correlated with poor prognosis and the reason for this is discussed.