Agar dilution minimal inhibitory concentrations (MICs) of penicillin, tetracycline, chloramphenicol, and clindamycin were determined using Wilkens-Chalgren agar for 1,266 clinical isolates of anaerobic bacteria. In addition, a reference strain of Bacteroides fragilis was repeatedly tested and demonstrated the precision of the technique. Fifty-six percent of our Bacteroides melaninogenicus strains were resistant (MIC 24.0 ,ug/ml) to penicillin. Resistance to this antibiotic was also seen among other anaerobes, but the results are more in accord with previous reports. Resistance to tetracycline (MIC 24.0 ,ug/ml) was found in 60% of our isolates. Chloramphenicol proved to be the most effective agent in vitro with only 2.0% of strains resistant (MIC 216 ,ig/ml). Only 5% of strains were resistant to clindamycin (MIC -8.0 ,ug/ml), and this included 10 isolates of B. fragilis and 4 of B. melaninogenicus. The incidence of resistance of anaerobic bacteria to these frequently used antibiotics is greater than previous reports and indicates the need for reliable susceptibility testing of anaerobic bacteria.Numerous techniques have been proposed during the last 10 years for susceptibility testing of anaerobic bacteria. These include disk diffusion (11, 24), broth-disk methods (10,16,26), and minimal inhibitory concentration (MIC) methods using broth dilution (14, 16) and agar dilution (23, 27; V. L. Sutter, Program Abstr. Intersci. Conf. Antimicrob. Agents Chemother. 16th, Chicago, Ill., abstr. no. 453, 1976). These procedures have been reviewed and evaluated (3,22). The agar dilution procedure has the greatest degree of precision and is recommended as a reference method for anaerobic susceptibility testing (19). A recent report compared various practical methods with the agar dilution reference method (13) and found similar results with all the methods. The relatively easy-toperform and reliable broth-disk procedures are the techniques that most clinical microbiology laboratories should have available. The choice of when to perform ousceptibility tests depends on the experience of the medical staff and the type of patients and procedures found in the institution.The qualitative results observed with the broth-disk technique contribute a limited amount of data to our overall information on the susceptibility of anaerobic bacteria and on the emergence of resistance among anaerobic bacteria, which is similar to what has occurred among the facultative anaerobic and aerobic bacteria. Therefore, larger institutions with the volume, facilities, and expertise in infectious and laboratory medicine should be doing quantitative testing on anaerobic bacteria. We report here our MIC information on a significant number of recently isolated anaerobic bacteria from clinical infections during a 2-year period.