The second international standard for tetracycline.

A second international standard has been established to replace the first International Standard for Tetracycline, stocks of which were depleted. The International Unit of tetracycline is defined as the activity contained in 0.00101833 mg of the second International Standard, corresponding to a potency of 982 IU/mg. These values were assigned on the basis of the results of an international collaborative assay carried out by laboratories in 19 countries. Most of the assays were by diffusion techniques; a few were by turbidimetric methods. Both procedures gave statistically valid assays but there was some heterogeneity of repeated estimations within a laboratory and also some heterogeneity of the mean estimates from each laboratory. The agreed defined potency was the unweighted mean of 281 individual assays; the confidence limits were 978-986 IU/mg.