Antibiotic sensitivity tests on most anaerobic organisms isolated from a given patient are seldom of use to that individual. This is because both isolation and sensitivity testing of this group of bacteria take so long. It is thus vital that the microbiologist should be able to supply to the clinician the cumulated results of past experience, ideally his own, but if not, then the experience of others, taking into account geographical variations whenever possible. Because of the possibility of geographical variation, it is important that every institution should carry out sensitivity testing on easily isolated and rapidly growing bacteria. Bacteroides fragilis is probably the most common of these, and because it is among the more resistant anaerobes and is present in the majority of anaerobic infections, it is often used as an "indicator" both of the diagnosis and of the likely response to a given drug regimen.