Seaprose is a semialkaline proteinase endowed with proteolytic effect and antiinflammatory activity tested in different clinical trials. There is clinical evidence that seaprose reduces sputum viscoelastic properties in chronic hypersecretory bronchitis. The present study evaluated (in a double-blind design vs. placebo) the activity of seaprose on bronchial inflammation, mucus glycoprotein secretion and bronchial humoral defence mechanism in chronic bronchitic patients clinically stable (10 per group). Markers of bronchial inflammation (albumin, albumin/total protein ratio) and bronchial infection (DNA), of mucus glycoproteins (fucose and N-acetylneuraminic acid) and of humoral defence mechanism (secretory-IgA) were tested in sputum. We found that ten-day treatment with seaprose (90 mg/day) reduced sputum albumin during the observation period, the difference being statistically significant at the 18th day. The sputum albumin/total protein ratio also decreased by 50% at the end of the study. In the same group, sputum DNA, secretory-IgA, fucose and N-acetylneuraminic acid remained unchanged after treatment. The placebo group did not show any significant changes in the sputum marker substances. This study provides experimental evidence for the antiinflammatory activity of seaprose on bronchial mucosa in chronic bronchitic patients studied in a stable phase of their disease. Furthermore the drug does not seem to affect mucus glycoprotein secretion or secretory-IgA production.