If the disc method of susceptibility testing used is that described by Bauer et al. (Am J Clin Path 45:493, 1966; Federal Register 37:20525-20529, 1972), a disc containing 10 mcg of gentamicin should give a zone of inhibition of 15 mm or more to indicate susceptibility of the infecting organism. Zones of 12 mm or less indicate that the infecting organism is likely to be resistant. Zones of greater than 12 mm and less than 15 mm indicate intermediate susceptibility. In certain conditions it may be desirable to do additional susceptibility testing by the tube or agar dilution method; gentamicin substance is available for this purpose.

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