For nearly a decade microbiologists have used the MIC breakpoints published in the BSAC Guide to Susceptibility Testing to interpret susceptibility. Historically, and unlike the rest of Europe, the UK and Ireland have used a comparative method of disc testing to interpret susceptibility rather than one based on a correlation between MIC and zone of inhibition. Although innovative when introduced in the 1970s, Stokes' comparative method has evolved ad hoc and it has become increasingly apparent that there is a need for a standardized method of disc testing that is correlated with BSAC MIC breakpoints. The method described here, like all other standardized methods of disc testing, cannot be adapted by the user, and interpretative criteria are only applicable if the method is adhered to fully. A major advantage of this approach to susceptibility testing is that data from several sources can be combined for surveillance of resistance, a task that has been made much easier by the introduction of this method and coincides with the availability of automated zone measuring devices. It is hoped that the method described here will provide the core document for standard operating procedures; however, changes will necessarily occur over time as the method is developed and refined.