The relative importance of host and bacteria-derived deoxyribonucleic acid (DNA) in the increased viscoelasticity of purulent sputum in cystic fibrosis (CF) and other airway diseases is unclear. We report the identification of the DNA associated with mucus glycoproteins purified from the purulent sputum of 9 patients with CF. Mucus glycoproteins were purified from CF sputum by gel exclusion chromatography and the co-purifying DNA isolated by phenol extraction. Electrophoresis indicated that the DNA preparations had a size of approximately 300 to greater than 50,000 bases. The origin of the DNA was determined by slot blotting and subsequent hybridization with 32P-labelled DNA probes specific for human DNA sequences and those from bacterial species commonly isolated from CF sputum. The results indicated that in all cases the DNA was almost entirely human in origin. This implies that it is the patient's own DNA which may contribute to the rheological abnormalities of CF sputum.