Negatively stained aggregates of antigen (polyoma or verruca vulgaris virus) and antibody (from rabbit or goat) were examined in the electron microscope. The antibody molecules appeared as cylindrical rods (often, but not always, showing a beaded appearance) with a long axis of 250 to 270 A and a short axis of 35 to 40 A. The combining sites were at the opposite short ends of the antibody molecules separated by the length of 250 to 270 A of the antibody molecule.

Aggregates of antigen and antibody showed regions of orderly arrangements and frequently ring structures of five or more linked virus particles. Sometimes a virus particle in the center of these ring structures was linked to the peripheral particles. In extreme antibody excess, cross-linking was only rarely observed and virus particles were surrounded by a dense aura of antibody molecules.

The specificity of the two combining sites of most antibody molecules is identical. This was utilized to examine the antigenic relation between the normal (icosahedral) and aberrant forms of polyoma virus.

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