SummaryIt has been known for some time that the viscosity of sputum from cystic fibrosis (CF) patients differs from that in other disease types characterized by pulmonary hypersecretion. However, it has never been established that there is in fact any abnormality in the rheologic properties of CF sputum. We have recently developed techniques to examine the rheology and mucociliary transportability of tracheal secretions from dogs utilizing the small quantities of mucus available from a healthy animal. In the present study, thesein vitrotechniques were applied to sputum samples obtained from 15 adult CF patients. For mucoid and mucopurulent CF sputum samples, the viscoelastic properties were remarkably similar to those observed for control canine tracheal mucus samples. For purulent CF sputum, elasticity and viscosity tended to be higher and the viscosity/elasticity ratio lower than for either the nonpurulent sputum or the canine tracheal mucus. The logarithm of elasticity at 1 rad/sec was 2.55 ± 0.35 (S.D.) for purulent sputum, 2.05 ± 0.49 for mucoid and mucopurulent sputum, and 2.17 ± 0.36 for canine tracheal mucus.From the viscoelastic properties of CF sputum, one would have predicted thein vitromucociliary transportability, at least for the nonpurulent samples, to be the same as that of canine tracheal mucus. However, the frog palate assay indicated a consistently lower rate of transport (mean difference, 13%) than that predicted from previous studies. This result would, therefore, suggest that if the prediction from canine tracheal mucus is applicable there is a factor in the CF sputum that results in a temporary inhibition of frog palate ciliary beating. This apparent inhibition of mucociliary transport is, however, relatively minor, and it is perhaps more remarkable that for none of the sputum samples collected was thein vitrociliary transport rate particularly low;i.e., observed values ranged from 65 to 100% of frog palate control.SpeculationThere is probably no rheologic abnormality in the tracheal mucus of patients with cystic fibrosis other than that associated with purulence. Cystic fibrosis sputum can be moved reasonably well by ciliary action, but there is, nevertheless, some suggestion of the presence of a ciliary inhibitory factor in it.