Forty outpatients with sinusitis were treated with equimolar doses of Ampicillin (556 mg) of Bacampicillin (800 mg) three times daily in a doubleblind fashion. To control the efficiency of the therapy the number of leukocytes, sedimentation rate, subjective and objective symptomes were evaluated before the first dose and on the 2nd and 10th day. On the 1st day nasal secretions were collected at the time the first dose was given and 1, 2, and 3 h later. In these samples the concentrations of the antibiotic, of protein and albumin were determined. The amount of nasal secretions collected as well as the protein and albumin content remains fairly constant during the experiment. The concentration of Bacampicillin in nasal secretions reaches its maximum of 0.92 microgram/ml 1 h after the application whereas with Ampicillin it is 0.59 microgram/ml after 2 h. These values correspond to the concentrations which were found in normal test persons. The clinical result of the therapy is slightly better with Bacampicillin only if patients with severe sinusitis were taken into account. In the patients with moderate or slight symptoms no difference was found. Two of the patients which were treated with Ampicillin had to stop the therapy because of severe diarrhoe. When using Bacampicillin no patients had diarrhoe. This study shows that Bacampicillin (as an inactive ester of Ampicillin) is resorbed quicker and gives higher concentrations in the nasal secretions. Thus, the therapeutic effect is better in severe cases as compared to Ampicillin. Moreover, the side effects are less.