Nesosteine, a new agent shown to improve physical characteristics of bronchial mucus in animal models, has been studied in chronic bronchitic patients to confirm its effect on viscoelasticity of bronchial secretions. Sputum was collected from hypersecretory bronchitic patients in a stable condition before and after seven days of treatment with nesosteine (900 mg/day) or a placebo. At the end of treatment a significant (p less than 0.05) decrease was found in the viscosity in the nesosteine group: on the contrary, a slight, non-significant increase in viscosity was observed in the mucus samples of the placebo group. The improvement in rheological characteristics of the bronchial mucus in the patients treated with nesosteine was associated with an increase in mucus transport rate (mucociliary clearance) observed in the same samples. The findings indicate that nesosteine reduces mucus viscosity in chronic bronchitic patients and that this change induces also an improvement in mucociliary clearance.