The serum and bronchial mucus of 16 patients with cystic fibrosis (aged 5 days to21 years) was tested for the presence of specific antibodies to lungs and pancreas. The gel diffusion technique of Ouchterlony was employed to demonstrate the antigen-antibody reaction. Antigens were prepared as crude tissue extracts of lung and pancreas obtained either at operation or within an hour after death from patients with and without cystic fibrosis. The mucus from 14 of the 16 patients with this disease demonstrated clear and distinct precipitin lines to the cystic fibrosis lung extract within 18 to 24 hours. The 2 nonreactive mucus specimens were taken from the 2 youngest patients. With the cystic fibrosis pancreas extract, however, the precipitin lines were vague and hazy and were not visible until after 48 to 72 hours of incubation. No precipitin lines were noted when tissue extracts of non-cystic fibrosis origin were used. Bronchial mucus from 26 non-cystic fibrosis subjects was similarly tested and failed to demonstrate any bands. Similar test methods with serum used in lieu of bronchial mucus failed to demonstrate any specificity. Such antibodies may be merely an interesting sequel to tissue destruction. However, because of the specificity of the reaction, it is possible that they may play a role in the pathogenesis and/or the chronicity of the disease.