Calf lymph vaccine, as prepared for smallpox vaccination, contains so much foreign matter that its virus content cannot be counted by the direct sedimentation methods. Extraction of the virus with fluorocarbon is described. In the miniature extraction apparatus, quantities as small as a few milligrams of vaccine are sufficient to yield enough virus for ready recognition and counting in the electron microscope. The method is applicable also to small bits of tissue whose virus particle content must be counted.Two lots of calf lymph vaccine have been studied. Their virus particle counts were 1.2 and 7.1 × 1010/ml. Both had met the minimum potency requirements of the National Institutes of Health.
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