Physicians must use empirical treatment initially for anaerobic infections. However, such treatment can be targeted if clinicians establish the nature of the infection and know the usual infecting flora of that type of infection and how the flora may have been modified by the use of antimicrobials. Physicians must also be aware of the usual susceptibility patterns of various anaerobes and nonanaerobes in the hospitals in which they work. Despite cost containment, it is still important to isolate all anaerobes present, to provide at least general identification (and specific identification of key organisms such as the Bacteroides fragilis group), and to keep the organisms alive so that they may be referred elsewhere for definitive identification and susceptibility testing, if indicated. Because resistance is increasing among anaerobes, susceptibility testing is very important. Susceptibility testing should be done when patients are seriously ill, when patients do not respond to therapy or relapse, when there are few data available on a species, when the organisms isolated are frequently resistant, and when patients require prolonged therapy. Periodic surveys of susceptibility patterns should be done on isolates from individual hospitals. Tests most useful for individual patient isolates are the Etest (AB BIODISK, Solna, Sweden), an expensive test, and the microbroth dilution test. Testing should be done on organisms that are the most virulent and most resistant to antimicrobial agents.