A medium for antimicrobial susceptibility testing with the diffusion method. This medium, used in the Bauer and Kirby technique, has been adopted by the National Committee for Clinical Laboratory Standards (NCCLS) in the USA as the definitive method for susceptibility testing. The medium has a very low thymine and thymidine content, which makes it suitable for testing trimethoprim and sulfonamide, which is checked to ensure correct zone sizes with aminoglycoside and tetracycline antibiotics.