Sedimentation counting of particles via electron microscopy
Electron microscope counts of virus particles have usually required highly concentrated, purified, preparations with a minimum of extraneous material (2, 4, 6). Isaacs has thoroughly reviewed these results recently (5). But, the virologist frequently does not require such purity. He generally deals with crude tissue extracts, plasmas, chorioallantoic fluids, etc., in which virus activity may be 106 to 109 ID50 per ml. It is rarely greater than 1010. This paper will describe electron microscope counting of two viruses in just such crude form.
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