Susceptibility testing is indicated for any organism that contributes to an infectious process warranting antimicrobial chemotherapy if its susceptibility cannot reliably be predicted from existing antibiograms. Antimicrobial resistance patterns for many anaerobic bacteria have changed significantly over the last several years, resulting in a lack of predictability for many species. Susceptibility testing of anaerobes is recommended for surveillance purposes and for specific clinical situations. Two end-point-determining susceptibility testing methods for anaerobic bacteria are described in Clinical and Laboratory Standards Institute document M11-A8 - Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard - Eighth Edition. The agar dilution method (Wadsworth) remains the reference standard, and is well suited for surveillance testing and research. It is also the standard to which other methods are compared. Broth microdilution is well suited for the clinical laboratory, but is currently limited to testing of Bacteroides fragilis group organisms and selected antimicrobial agents. QC criteria for each procedure are also described. The tables in CLSI document M100,1 when used in conjunction with this standard, represent the most current information for drug selection, interpretation, QC, and antibiogram reports using the procedures standardized in M11. Users should replace tables published in earlier standards with these new tables. (Changes in the tables since the previous edition appear in boldface type). When new problems are recognized, or improvements in these criteria are made, changes will be incorporated into future editions of this standard and also distributed as informational supplements.