The susceptibility of anaerobic bacteria is determined by one of two methods: by using data from surveillance studies or by performing in vitro susceptibility tests. In the first method, surveillance studies of the activity of various antimicrobial agents against large collections of strains of different species are performed at a reference laboratory; in the second, in vitro susceptibility tests of clinically relevant anaerobic isolates are performed so that therapeutic guidance can be given. Susceptibility testing can be done by the dilution or diffusion methods, and the method chosen is dependent on the purpose of the assay. The agar dilution method is the NCCLS reference method and is well suited for surveillance studies. The broth microdilution method is recommended for routine susceptibility testing of anaerobes, but the E test--which has an antibiotic gradient and is used on agar plates--also seems to be useful for routine susceptibility testing of anaerobes. The guidelines with regard to methods for susceptibility testing of anaerobes and determining breakpoints of antimicrobial agents for the European countries should be uniform with those for the United States.