There have been numerous reports on resistance of anaerobic bacteria against antimicrobial agents. Therefore, to assess the situation in Zurich, 187 anaerobic strains of various bacterial genera, isolated from clinical specimens during winter 1990/91, were tested for their susceptibility to antimicrobial agents active against anaerobic bacteria. Besides the Bacteroides fragilis group, which is naturally resistant against penicillin, 30% of isolates of other Bacteroides species were also resistant against penicillin. In general, anaerobes have remained susceptible to cefoxitin, chloramphenicol, clindamycin, imipenem, the 5-nitroimidazoles (metronidazole, ornidazole) as well as combinations of beta-lactam antibiotics with beta-lactamase inhibitors (clavulanic acid, sulbactam and tazobactam). Because rare strains resistant against cefoxitin, clindamycin and beta-lactams plus beta-lactamase inhibitors can be found, at least isolates from specific clinical situations should be tested for antimicrobial susceptibility. These are strains isolated from patients with brain abscess, endocarditis, osteomyelitis, arthritis, infected implants and prosthesis as well as those from persisting or recurrent bacteremia. Because the agar diffusion test yields unreliable results, minimal inhibitory concentration should be determined. Maybe the new 'E test' or the spiral gradient procedure can be used after evaluation.