Iso-Sensitest Agar is a medium developed specifically for anti-microbial susceptibility tests like the Disc Diffusion Method. This is a well-defined medium containing a stabilized mineral content to avoid antagonism to certain antimicrobials by metal ions. It allows the growth of the great majority of microorganisms without further supplementation. It can also be supplemented for fastidious organisms with 5% whole horse blood or 5% whole horse blood and 20mg/l beta-nicotinamide adenine dinucleotide (NAD). Some mutant strains which are totally dependent on thymine and thymidine for their growth will not grow on Iso-Sensitest Agar which has these two components at very low levels. Care must be taken to recognize these strains. Certain supplements interefere with anti-microbial activity and tests must be performed to measure their effect. These supplements and the anti-microbial agents affected (in parentheses) include: thymidine (trimethoprim), blood (sulfonamides, trimethoprim, aminogly