The in vitro susceptibility to thiamphenicol and chloramphenicol of 272 anaerobes, most of which were recent clinical [human] isolates, was determined by broth dilution tests. With chloramphenicol, 133 anaerobic gram-negative non-sporing rods (48 Bacteroides fragilis, 13 B. thetaiotaomicron, 14 B. oralis, 16 Sphaerophorus varius etc.) had minimum inhibitory concentration (MIC) values of 0.03 through 16 .mu.g/ml. Very similar results (MIC, 0.06-16 .mu.g/ml) were obtained with thiamphenicol. In concentrations of 4 .mu.g/ml or less chloramphenicol inhibited 90.2% and thiamphenicol 78.95% of the strains. Strains with only moderate sensitivity to both antibiotics (MIC, 8 or 16 .mu.g/ml) belonged to B. fragilis or other Bacteroides species. Members of the Fusobacterium and Sphaerophorus group were susceptible to .ltoreq. 2 .mu.g chloramphenicol/ml and .ltoreq. 4 .mu.g thiamphenicol/ml respectively. With both antibiotics, 102 strains of gram-positive non-sporing anaerobes (Propionibacterium acnes, Peptostreptococcus spp., Peptococcus spp.) were susceptible to .ltoreq. 8 .mu.g/ml. Of 37 Clostridium isolates, 35 (belonging to C. perfringens, C. septicum, C. cadaveris etc.) were inhibited by concentrations of 8 .mu.g/ml or less of chloramphenicol and thiamphenicol. Only 1 strain each of C. perfringens and Clostridium sp. had an MIC of 16 .mu.g thiamphenicol/ml. Accordingly, resistance to thiamphencol or chloramphenicol was not observed. A standardized monodisc agar diffusion test was performed on 40 Bacteroidaceae, 18 Peptococcaceae and 20 Propionibacterium acnes strains. Only a poor correlation was observed between MIC and zone size for thiamphenicol and chloramphenicol. Therefore, the inhibition zone diameter measurement cannot be regarded as a reliable method to detect chloramphenicol or thiamphenicol resistance in anaerobes. Thiamphenicol, which is virtually as active against anaerobes as chloramphenicol but lacks serious toxicity, may well play an important role in the therapy of various anaerobic infections.