In dentistry antimicrobials are used in the treatment of progressive periodontitis and odontogenic abscesses, therefore the susceptibility to commonly used antibiotics of capnophilic and anaerobic species causing these diseases should be investigated. The activity of penicillin, amoxycillin, cefoxitin, clindamycin, doxycycline, metronidazole and ciprofloxacin was investigated. One hundred and sixty four isolates from subgingival plaque samples of 66 patients with progressive periodontitis and 192 bacterial strains from pus of 74 patients with odontogenic abscesses were included in this study. The majority of species tested were gram-negative anaerobes (Prevotella spp., Porphyromonas spp., Fusobacterium spp.), and were highly susceptible to clindamycin and metronidazole. Nearly 6% of the periodontal isolates and 22% of the bacteria obtained from pus samples produced beta-lactamases. With the exception of the periodontopathogenic species Actinobacillus actinomycetemcomitans and Eikenella corrodens, clindamycin seemed to be a useful antibiotic and could be recommended for empirical antimicrobial treatment.