Reference data on the overall resistance patterns of anaerobic bacteria are needed since in individual clinical cases susceptibility testing is complicated by the long generation time and difficulties of test conditions. Benzylpenicillin and tetracyclines are the drugs of choice against Clostridium perfringens. Newer cephalosporins are also active, whereas other Clostridium species have been found resistant. Beta-lactam antibiotics show high activity against Peptococcaceae; resistance to clindamycin should be noted, however. The good in vitro activity of tetracyclines, clindamycin and erythromycin against Propionibacterium acnes could be confirmed in the systemic and local therapy of acne vulgaris. In contrast to gram-positive anaerobic bacteria, only some penicillin and cephalosporin derivatives are sufficiently active against gram-negative rods of the Bacteroides group. Sporadic resistance is seen to the newer tetrayclines, nitroimidazole compounds and to lincomycins. Nevertheless, these substances must be considered as standard therapeutics for infections caused by anaerobic gram-negative bacteria. It should be mentioned that cefoxitin may offer new therapeutic possibilities.