The activity of an antibiotic can be demonstrated under suitable conditions by its inhibitory effect on organisms. Antibiotic assays are performed by the cylinder plate method and the turbidimetric assay. These media has been formulated according to the specifications and procedures listed by the Food and Drug Administration, European Pharmacopeia, Code of Federal Regulations, and the USP. The Antibiotic Media are identified numerically and also, where applicable, with names assigned by Grove and Randall in Assay Methods of Antibiotics. Antibiotic Medium #11 is recommended for determing antibotic potency to neymycin.