Unsuccessful medical therapy for treatment of acute tonsillitis frequently results in onset of recurrent or chronic forms rendering surgical treatment necessary. We have studied some of the factors involved in the evolution of these chronic or recurrent forms and, in particular, the distribution of different bacterial strains in tonsillar tissue, their response to treatment with antibiotics and mechanisms of bacterial resistance. The distribution of bacterial flora (saprophytic and pathogenic) present on the surface of the tonsils, in the crypts and in the tonsillar tissue was studied in a control population of 30 children (20 boys and 10 girls) aged between 2-13 years, all affected with chronic tonsillitis and submitted to tonsillectomy. The same study was performed in a group of 80 sex and aged matched children, also presenting with chronic or recurrent tonsillitis and treated with amoxicillin or amoxicillin-clavulanic acid or cefaclor or clarithromycin 72 h before surgery. The 80 subjects were randomly divided into four groups so that each antibiotic was tested on 20 subjects. The distribution of the bacterial population on the surface of tonsillar tissue, in the crypts and in the deeper tonsillar tissue is of particular interest concerning the affinity of bacteria to the different tissue areas. In particular the interaction between crypt and tonsillar core, which could be a factor involved in the process of worsening of bacterial infection in the tonsils, is evidenced. The four antibodies tested showed different abilities to eradicate infection: Haemophilus influenzae was found to be the most resistant germ to antibiotic therapy and was thus the most frequent cause of recurrent infections. The characteristics and the mechanisms of adherence and resistance to beta lactam antibiotics were also analysed.