To investigate the genetic identities of the mucins secreted in cystic fibrosis (CF) airways, sputum was collected from five individuals. Samples were separated into gel and sol phases by high-speed centrifugation and the gel phase was extracted in 6 M guanidinium chloride. The ‘insoluble’ residue remaining after extraction of the gel phase was brought into solution by reduction/alkylation. Density-gradient centrifugation in CsCl revealed polydisperse distributions of sialic acid-containing mucins in the gel phase, insoluble residue and sol phase fractions and the degree of variation between the different individuals was low. Antibodies recognizing MUC5AC and MUC5B identified these mucins in each of the fractions. MUC2, however, was present only as a component of the insoluble residue from the gel which accounted for less than 4% by mass of the total mucins. MUC5B and MUC5AC from the gel phase were large oligomeric species composed of disulphide-bond linked subunits and MUC5B was present as two populations with different charge densities which are likely to correspond to MUC5B ‘glycoforms’. The sol phase contained, in addition to MUC5AC and MUC5B, mainly smaller mucins which did not react with the antisera and which were probably degraded. MUC5AC appeared to be enriched in the sol, suggesting that this mucin may be more susceptible to proteolytic degradation than MUC5B. The mucins present in sputum remained broadly similar during acute exacerbation and following antibiotic treatment, although the relative amount of an acidic MUC5B glycoform was decreased during infection.