To investigate the characteristics of bronchial mucus in bronchopneumopathic patients before and after stepronine-lysine salt treatment, a rheological double-blind randomised study has been performed. The assessment of the rheological behaviour of bronchial secretions was performed using a coaxial cylinder rheometer set up for dynamic test with sinusoidal oscillation. While the placebo group did not show significant modifications in the viscosity and elasticity parameters, in the stepronine group a significant viscosity reduction was found.