A comparison of the weight and photometric methods of primary assay of BCG vaccine has been made, using a vaccine prepared in albumin-free medium but containing Tween 80. In the weight method, the bacteria were trapped on a membrane filter; for photometry a Pulfrich Elpho photometer and an instrument of Czech origin were used. The photometric results were the more precise, provided that the measurements were made within two days of completion of growth; after this time the optical density of the suspension began to decrease slowly. The lack of precision of the weighing method is probably due to the small weight of culture deposit (which was almost on the limit of accuracy of the analytical balance) and to difficulties in the manipulation of the ultrafilter
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