The International Reference Preparation for Opacity; notes and description.

The International Reference Preparation for Opacity has been established by the WHO Expert Committee on Biological Standardization. The preparation is a suspension of Pyrex-glass particles of approximately the size of bacteria, and is similar to the standard for opacity used by the National Institutes of Health (Public Health Service), Bethesda, Md., USA (which have supplied it). The preparation, to which has been assigned an opacity of 10 International Units per millilitre, may be used for the direct visual estimation of the opacity of bacterial suspensions. It is held, for distribution to national laboratories for biological standards, by the Department of Biological Standards, Statens Seruminstitut, Copenhagen, Denmark.