The international standard for bacitracin.

In 1952, the Department of Biological Standards, National Institute for Medical Research, London, was authorized by the WHO Expert Committee on Biological Standardization to proceed with the establishment of an International Standard for Bacitracin. Thirty grams of bacitracin were obtained and it was decided to compare the proposed International Standard with the existing Working Standard for Bacitracin of the United States Food and Drug Administration.Six laboratories in five countries participated in the collaborative assay. A total of 52 assays was carried out, but the overall weighted mean potency was calculated from only the 48 biological assays, the 4 assays by the Hiscox method being excluded. The biological assays were done by the plate, the cylinder, or the disc method, using Streptococcus pyogenes, Staphylococcus aureus, Corynebacterium xerosis, or Micrococcus flavus.After analysing the results by standard statistical methods, the authors established the batch of bacitracin examined as the International Standard. The Standard contains 55 International Units of activity per mg, and the International Unit has therefore been defined as the activity contained in 0.0182 mg of the International Standard for Bacitracin.