Twenty four patients with acute infective exacerbations of chronic bronchitis received amoxycillin alone or in combination with erdosteine (a mucolytic agent) for a week in a double blind, placebo controlled study. Clinical assessment scores, body temperature, serum and sputum amoxycillin concentrations, and sputum culture results were recorded in each group. Erdosteine significantly increased antibiotic concentrations in sputum but not in serum. The combined treatment also caused a more rapid decrease in sputum viscosity and in body temperature and faster sterilisation of the sputum. These results show that erdosteine increases amoxycillin concentration in sputum in patients with acute exacerbations of chronic bronchitis. This effect may be due to a reduction in the viscosity of the bronchial secretions produced by erdosteine.