In vivo and in vitro mucociliary clearance have been investigated in 27 patients with chronic bronchitis with relation to the rheological, cytobacteriological and biochemical characteristics of sputum. In normal subjects, the mean percentage of bronchial radioactivity cleared (PBRC) by mucociliary clearance was 30.1 +/- SD: 12.5. The patients studied were differentiated into two groups: 1) 10 patients showing a PBRC similar to that of the normal subjects (30.6 +/- SD: 14.3), 2) 17 patients with a markedly decreased mucociliary clearance (PBRC: 4.1 +/- SD: 4.2). In the healthy subjects, PBRC by coughing was significantly lower than by mucociliary clearance, while no consistent difference was observed in the patients. A weak but significant positive correlation (r = 0.53, P less than 0.01) was observed between in vivo mucociliary clearance and the relative transport rate of the sputum measured on the ciliated frog palate mucosa. The optimal rheological conditions for mucociliary transport were characterized by an apparent viscosity (eta 0) ranging from 25 to 180 poise, a strain recovery (SR) ranging from 4 to 12 units and an elastic modulus (G) ranging from 4 to 8 dyn/cm-2. The apparent viscosity and the elastic modulus G respectively, were significantly correlated with the concentration of serum albumin and of secretory IgA. Patients with a sputum leukocyte count higher than 3500/mm3 had a significantly decreased mucociliary clearance in comparison with non-purulent sputum patients.