Pus obtained by needle aspiration of 91 peritonsillar abscesses was examined microbiologically. A positive culture was obtained in 55 patients (60 per cent). Sixty-four bacteriological isolates were grown. Forty patients had a pure growth of a single organism, of which 21 (53 per cent) were beta Haemolytic streptococci. Pure growths of Staphylococcus aureus were found in only three patients. Fifteen patients had mixed organisms, including anaerobes, in their pus and the resistance to penicillin was low. Only the bacteroides species were generally penicillin resistant. The vast majority of patients made a good recovery following needle drainage of the abscess and treatment with parenteral penicillin. The patients with a mixture of penicillin sensitive and penicillin resistant organisms also made a good clinical recovery following needle drainage and administration of parenteral penicillin. The relevance of these findings in the pathogenesis and management of peritonsillar sepsis is discussed.