Clinical studies have emphasized the importance of anaerobic bacteria in recurrent tonsillitis. These bacteria are usually sensitive to penicillin. However, recent reports have demonstrated that penicillin-resistant Bacteroides strains can be isolated from inflamed tonsils. These strains are resistant to penicillin due to β-lactamase production and the strains may therefore be responsible for the failure of penicillin in the treatment of recurrent tonsillitis. Ten patients with recurrent tonsillitis were given 1 g tinidazole every 24 h for 7 days, 21 patients 0·15 g clindamycin every 6 h for 10 days and 19 patients 1 g phenoxymethylpenicillin every 12 h for 10 days. Tonsillar specimens were taken before and after antibiotic treatment. Tinidazole eliminated β-lactamase-producing Bacteroides strains in eight of nine patients. Five patients harboured β-lactamase-producing Staphylococcus aureus both before and after treatment. Clindamycin eradicated β-lactamase-producing Staph. aureus in nine of ten patients and Bacteroides strains in 10 of 14 patients. Penicillin failed to eliminate Staph . aureus in 11 patients. β-lactamase-producing Bacteroides strains were recovered in nine patients after therapy.