1. A very active filtrate can be obtained from vaccinia lesions by grinding up the fresh tissue with glass fragments, emulsifying in hormone broth, centrifuging the emulsion and filtering the supernatant fluid through a Berkefeld V filter.

2. The sterile filtrate so obtained has been shown by comparative titration on rabbits to have about one-sixteenth of the activity of the non-sterile emulsion used in human vaccination.

3. Centrifugation of such a filtrate shows a partial concentration of the virus in the lowermost layer.

4. The virus survives for a long time, if the filtrate is kept near the freezing point, and probably will survive indefinitely if kept frozen. The addition of glycerine is not necessary.

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