The ecosystem of the dental plaque in periodontal diseases is very complex: the study of such micro-organisms, which are mostly strict anaerobes, requires the use of specific techniques under conditions of strict anaerobiosis. The aim of the present study was to design a rapid method to evaluate the activity of antimicrobials on mixed bacterial plaque of subjects with periodontal diseases. The study was carried out using a computerised instrument generally used for simultaneous diagnostic tests with aerobic bacteria. Operative and methodological modifications were made to obtain conditions of strict anaerobiosis and the balanced growth of all the microbial forms present in the mixed cultures of the plaque. Penicillins and cephalosporins were active on all the samples, whereas colistin, gentamicin, kanamycin and nalidixic acid showed no activity. Clindamycin, tetracycline, erythromycin and penicillin G were effective only against some samples. The activity of the antimicrobials towards isolated strains was analogous to that towards the corresponding mixed culture.