In the complex aetiology of human actinomykoses, various Actinomyces species, especially A. israelii, play the pathogenetically leading role in the mixed flora so typical for these diseases. A number of other microaerophilic and anaerobic bacteria are also part of such mixed flora. Since these "concomitant" bacteria are themselves potentially pathogenic, it is important to have data on antibiotic sensitivity for the entire spectrum of microorganisms, to achieve effective and economic chemotherapy of human actinomykoses. Minimal inhibitory concentrations of antibiotics were determined for Actinomyces and the most important concomitant bacteria from actinomykotic processes: Actinobacillus actinomycetem-commitants, Bacteroides of the Melaninogenicus group, B. fragilis, B. theta-iotaomicron, Fusobacterium nucleatum, Fusobacterium spp. and Propionibacteria. The results indicate that aminopenicillines are the drug of first choice, if in the actinomykoses there are no Bacteroides strains resistant to beta-lactam antibiotics. These tests further indicate that clinically the combination of aminopenicillin and clindamycin, as well as aminopenicillin and metronidazol, are indicated. Minocycline and especially cefoxitin, a beta-lactamase resistant cephalosporin, inhibit with few exceptions all bacteria which are potentially involved in actinomykotic processes.