Periodontally affected sites may show resistance to mechanical therapy associated with conventional treatment. This periodontal situation is the result of the cooperation of bacteria, and in some cases the development of superinfections due to opportunistic bacteria is possible. These situations need particular attention in the choice of antibiotics. From some sites, bacteria not commonly found as periodontal pathogens may be isolated; they are generally resistant to antibiotics administered in periodontal therapy. Attention should be used particularly in cases where lesions most resemble those characteristic of the action of gram-negative bacteria. According to results of the present research these cases should be managed with antibiotic combinations and conventional mechanical therapy.