A drug-resistant strain of Mycobacterium tuberculosis is defined as one differing from the tight distribution of wild strains that have not come into contact with the drug concerned. Sensitivity tests are performed by the absolute concentration method, the resistance ratio method or the proportion method. The hypothesis underlying the proportion method is that there are appreciable differences in inoculum size so that there should be an association between the proportion on drug-free medium and the proportion on drug-containing medium. This hypothesis was not supported by a study on ethionamide-resistant strains. It indicated that variation in the proportion on drug-free medium was due to clumping of the bacilli in the inoculum rather than to differences in the number of bacilli. Hence, the use of the proportion method introduces errors in susceptibility testing. While the method can produce reliable results, it is more time consuming than a minimal inhibitory concentration determination, and should not be adopted as a standard method.