A medium for antimicrobial susceptibility testing to antibiotics using 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 a definitive method for susceptibility testing. The medium has a very low thymine and thymidine content, which makes it suitable for trimethoprim and sulfonamide testing, checked to ensure proper zone sizes with aminoglycoside and tetracycline antibiotics.