It has recently been suggested that in cystic fibrosis (CF), there is no rheological abnormality of airway secretions other than that associated with purulence, and that the apparent inhibition in the mucociliary transport rate might be partly due to a ciliary inhibitor present in these secretions. In order to ascertain this assumption, expectorated airway secretions were collected without salivary contamination in twenty-four CF patients and the rheological properties were measured. Using a photometric method, the effects of CF sputum samples were analysed on the ciliary beat frequency (Fm) of the frog palate, and we measured their mucociliary transport rate (TR). In all but one CF sputum, TR and Fm were lower than that of the control frog mucus (median TR: 18.7 and 11.6 mm min-1; median Fm: 12.3 and 11.3 Hz, respectively). In the eighteen patients in whom the rheological properties were outside the range for optimal mucociliary transport, the clinical Shwachman score was significantly (P less than 0.05) lower (median score: 66.2 points) than in the six patients with optimal rheologic properties (median score: 73 points). In the eleven CF patients with superinfection, the apparent viscosity (eta o) was significantly higher (P less than 0.01; median eta o: 24.4 Pa. s) and TR, expressed as a percentage of the reference value, was significantly lower (P less than 0.05; median Tr: 54.5%) in comparison with the values obtained for the thirteen non-superinfected CF patients (median eta o: 15 Pa. s and median TR: 66% respectively). The CF patients with markedly hyperviscous sputum (eta o higher than 30 Pa. s) exhibited a low Shwachman score.