Twenty patients with stable chronic bronchitis entered a double-blind study in which changes in clinical and respiratory function and biochemical and rheological variations were examined after treatment with the mucoregulator S-carboxy-methyl-cysteine (S.C.M.C.). After one week of single-blind placebo, a two week double-blind study was initiated with placebo or oral S.C.M.C. 3 g/24h. After two weeks, a significant clinical improvement was observed in patients treated with S.C.M.C. During treatment, there was no change in respiratory function, although a drop in FEV1/VC was noted in the placebo group. A significant increase in the viscosity of the expectorations was observed after treatment with S.C.M.C. for two weeks. The levels of secretory IgA and of serum albumin in the expectorations remained stable, whereas in the placebo group, there was a slight but significant increase in serum albumin. In this group of non-infected chronic bronchitic patients, S.C.M.C. appeared to normalize the secretory function of the bronchial mucosa by preventing inflammation and enhancing the viscoelastic properties of bronchial secretions.