Virus-free filtrate, obtained from suspensions of vaccine virus-infected dermal pulp of rabbits and rich in the soluble substances of vaccinia, was shown to contain four distinct components in electrophoresis experiments. Electrophoretic and serological observations served as a guide in developing a method for separating these components from one another. This method depended upon changes in the solubilities of the components with alterations of pH.
Three of the four components appeared to be serologically inert when tested with anti-vaccinia sera. All of the L- and S-activity was found to be associated with a single component which was electrically homogeneous at several values of pH and which was homogeneous in the ultracentrifuge.
This single substance, designated as LS-antigen, precipitates in equal titers with optimal amounts of L- and of S-antibody and is completely removed from solution by absorption with either antibody.
The LS-antigen of vaccinia appears to be a protein molecule with two antigenically distinct parts, L and S. Heating modifies the L-portion in such a manner that the substance no longer precipitates with L-antibody; this degraded antigen still combines with L-antibody, as is shown by inhibition tests, and still precipitates with S-antibody. Similarly, treatment with heat and dilute alkali modifies the S-portion of LS-antigen so that it combines but does not precipitate with S-antibody; and at the same time all recognizable immunological properties of the L-portion are destroyed.