THE SECOND INTERNATIONAL STANDARD FOR BACITRACIN.

The National Institute for Medical Research, London, was requested by the WHO Expert Committee on Biological Standardization to establish a second International Standard for Bacitracin since stocks of the first International Standard were running low. A batch of 500 g of zinc bacitracin was obtained in 1962 and was distributed into ampoules in 100-mg amounts and dried in vacuum; the ampoules were then filled with dry nitrogen and sealed. This proposed replacement material has been assayed biologically in terms of the first International Standard in seven laboratories in five countries by means of plate diffusion assays.In spite of a large difference in purity between the first and proposed second standards, the variation in the potencies obtained for the proposed second standard were small, although significant. This material has been established as the second International Standard for Bacitracin with a defined potency of 74 mg. The International Unit of Bacitracin is defined as the activity of 0.01351 mg of the second International Standard for Bacitracin.