The international standard for novobiocin.

The National Institute for Medical Research, London, was requested by the WHO Expert Committee on Biological Standardization to arrange a collaborative assay of the International Reference Preparation of Novobiocin in order to define the International Unit of Novobiocin. The collaborative assay showed that the International Reference Preparation was not suitable for use as an international standard, and a second preparation of 400 g of novobiocin acid was obtained in 1962. This second preparation was distributed into ampoules in 100-mg amounts and dried in vacuum. The ampoules were then filled with dry nitrogen and sealed. This material-the proposed international standard for novobiocin-has been assayed biologically in eight laboratories in six countries by means of plate diffusion assays; it has been assayed against the working standard preparation of the US Food and Drug Administration.The variation in the potencies obtained for the second proposed international standard for novobiocin were small and not significant. The material has been established as the International Standard for Novobiocin with a defined potency of 970 International Units per mg, and the International Unit has been defined as the activity contained in 0.001031 mg of the International Standard.