30 patients with recurrent tonsillitis were treated with amoxycillin/clavulanic acid (500 mg/125 mg) t.i.d. for 10 days. The most often isolated potential aerobic pathogenic bacteria were Staphylococcus aureus, Streptococcus milleri and Haemophilus parainfluenzae. Group A streptococci were isolated from 7 patients. Anaerobic cocci and Bacteroides species were the dominating anaerobic bacteria isolated from the tonsils. Tonsillar cultures were taken before antibiotic treatment started, on days 11-12, day 30, and day 90. Beta-lactamase producing aerobic and anaerobic bacteria were present in 13 patients prior to treatment, on days 11-12 in 18 patients, on day 30 in 15 patients, and on day 90 in 13 patients. Group A streptococci were eradicated in 6/7 patients. In the aerobic tonsillar microflora, the numbers of viridans streptococci decreased during treatment but were normalized after 30 days. Only minor changes in the numbers of other aerobic microorganisms occurred during the investigation. The effect on the anaerobic microflora was minor and no new colonizing microorganisms were isolated during or after antibiotic treatment. All patients except one were cured on days 11-12. Five patients had another episode of tonsillitis during the observation period of 3 months. The antibiotic treatment was well tolerated in most patients and mild adverse events such as nausea, vomiting and diarrhoea were observed in 3 patients.