It has been shown experimentally that mixtures of two types of particles, namely, elementary bodies of vaccinia and collodion particles coated with protein, sediment with a single boundary in the analytical centrifuge. Such mixtures have been shown to develop one or two boundaries on electrophoresis in the Tiselius apparatus, depending on the type of coating on the surface of the collodion particles. When covered with the heat-stable soluble antigen of vaccinia, collodion particles migrate in the electrical field at the same rate as elementary bodies. On the other hand, if they are coated with a component of normal rabbit serum, they migrate at a different rate.

The estimation of purity of preparations of virus by means of data obtained by ultracentrifugation and electrophoresis is discussed.

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