A technique has been devised for obtaining free or uncombined pneumonia virus of mice (PVM). Free PVM, liberated from infected mouse lungs by means of this technique, is infectious and causes hemagglutination directly. The results of quantitative studies carried out in the high speed angle centrifuge indicate that the free virus is relatively small, with dimensions of the order of 40 millimicrons. When it is in combination with lung tissue particles, PVM appears to be a relatively large virus with minimal dimensions of the order of 140 millimicrons. Non-infectious virus particles, released from combination with lung tissue particles by heating, are similar to the infectious free virus in size.

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