In recent years, significant resistance to antimicrobial agents has been encountered among certain anaerobic bacteria. Susceptibility patterns vary from region to region, but even within a given region susceptibility is not always predictable. Initially, therapy of mixed anaerobic infections must be empirical, based on the nature of the infection, the usual flora of such infections, anticipated modification of this flora by pathophysiologic processes or prior antimicrobial therapy, and evaluation of Gram stains from appropriate specimens. If the infection does not respond well or the patient requires long-term therapy, antimicrobial susceptibility testing may be indicated in order to provide optimum therapy. Susceptibility testing is also indicated for determination of the usual patterns in a particular hospital, for monitoring geographical patterns, and to determine the activity of new antimicrobial agents.