Most media used for susceptibility testing contain sulfonamide inhibitors that make them unacceptable for testing sulfonamides. The major substance that inhibits sulfonamides has been identified as thymidine, and recent efforts to remove it from Mueller-Hinton medium have made it possible to perform susceptibility tests with sulfamethoxazole-trimethoprim by broth microdilution. Some lots of Mueller-Hinton broth are thymidine free, but some contain small amounts of thymidine and require the addition of thymidine phosphorylase or lysed horse blood which contains thymidine phosphorylase. Some lots may contain too much thymidine so that its activity cannot be reversed by adding the recommended amont of thymidine phosphorylase. Therefore the suitability of a medium must be determined by testing with control strains. Some organism, for example enterococci, cannot be tested in thymidine phosphorylase-treated media because thymine works in the same way thymidine does to inhibit the action of sulfamethoxazole-trimethoprim.