Ultracentrifugal studies on the CL dermal strain of vaccine virus indicate the following characteristics of the elementary bodies:—

1. A stable suspension of Paschen bodies in a dilute buffer solution of pH 6.2 to 8 sediments with the formation of a characteristic primary boundary which consistently shows a spread of approximately 14 per cent.

2. The principal sedimentation boundary is accompanied frequently by one or several more rapidly moving boundaries which probably are produced by groups of agglutinated elementary bodies consisting of two or more particles.

3. Occasionally the principal boundary may exhibit an irregular or peculiar behavior, a fact which necessitates a careful selection of material and the performance of many experiments for accurate interpretation of results.

4. The sedimentation constant of the slowest moving particles forming the principal boundary is computed to be 49.1 x 10–11 cm./sec./dyne. On the basis of this sedimentation rate, the average diameter of the smallest virus particles in appreciable amounts is estimated at 236 mµ. If the boundary spread is due principally to slight differences in particle size, then the largest single elementary bodies are approximately 252 mµ in diameter.

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