Nesosteine is a mucoregulatory agent. Nesosteine reduced the viscosity of tracheobronchial mucus in rabbits made bronchitic by H2SO4 aerosol and markedly increased mucoproduction in healthy animals. Nesosteine was more active than the best known mucolytic/mucoregulatory drugs, such as sobrerol, N-acetylcysteine and mercaptopropionylglycine. The fluidifying activity of the drug was also demonstrated in vitro (pig's gastric mucin), although this proved to be less marked than in vivo. Nesosteine reduced the amount of total proteins of the tracheobronchial mucus, acting on albumin, alpha 1, alpha 2, beta and gamma mucoproteins.